core/src/main/java/org/apache/brooklyn/core/workflow/ShorthandProcessorEpToQst.java [136:238]:
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                } else {
                    // shouldn't come here
                    return Maybe.absent("Input has trailing characters after template is matched: '" + inputRemaining + "'");
                }
            }
            return Maybe.of(result);
        }

        protected Maybe<Object> doCall() {
            boolean isEndOfOptional = false;
            outer: while (true) {
                if (isEndOfOptional) {
                    if (optionalDepth <= 0) {
                        throw new IllegalStateException("Unexpected optional block closure");
                    }
                    optionalDepth--;
                    if (optionalSkippingInput>0) {
                        // we were in a block where we skipped something optional because it couldn't be matched; outer parser is now canonical,
                        // and should stop skipping
                        return Maybe.of(true);
                    }
                    isEndOfOptional = false;
                }

                if (templateTokens.isEmpty()) {
                    if (Strings.isNonBlank(inputRemaining) && valueUpdater==null) {
                        return Maybe.absent("Input has trailing characters after template is matched: '" + inputRemaining + "'");
                    }
                    if (optionalDepth>0)
                        return Maybe.absent("Mismatched optional marker in template");
                    return Maybe.of(true);
                }
                String t = templateTokens.remove(0);

                if (t.startsWith("[")) {
                    t = t.substring(1);
                    if (!t.isEmpty()) {
                        templateTokens.add(0, t);
                    }
                    String optionalPresentVar = null;
                    if (!templateTokens.isEmpty() && templateTokens.get(0).startsWith("?")) {
                        String v = templateTokens.remove(0);
                        if (v.startsWith("?${") && v.endsWith("}")) {
                            optionalPresentVar = v.substring(3, v.length() - 1);
                        } else {
                            throw new IllegalStateException("? after [ should indicate optional presence variable using syntax '?${var}', not '"+v+"'");
                        }
                    }
                    Maybe<Object> cr;
                    if (optionalSkippingInput<=0) {
                        // make a deep copy so that valueUpdater writes get replayed
                        Map<String, Object> backupResult = (Map) CollectionMerger.builder().deep(true).build().merge(MutableMap.of(), result);
                        Consumer<String> backupValueUpdater = valueUpdater;
                        String backupInputRemaining = inputRemaining;
                        List<String> backupTemplateTokens = MutableList.copyOf(templateTokens);
                        int oldDepth = optionalDepth;
                        int oldSkippingDepth = optionalSkippingInput;

                        optionalDepth++;
                        cr = doCall();
                        if (cr.isPresent()) {
                            // succeeded
                            if (optionalPresentVar!=null) result.put(optionalPresentVar, true);
                            continue;

                        } else {
                            // restore
                            result = backupResult;
                            valueUpdater = backupValueUpdater;
                            if (optionalPresentVar!=null) result.put(optionalPresentVar, false);
                            inputRemaining = backupInputRemaining;
                            templateTokens.clear();
                            templateTokens.addAll(backupTemplateTokens);
                            optionalDepth = oldDepth;
                            optionalSkippingInput = oldSkippingDepth;

                            optionalSkippingInput++;
                            optionalDepth++;
                            cr = doCall();
                            if (cr.isPresent()) {
                                optionalSkippingInput--;
                                continue;
                            }
                        }
                    } else {
                        if (optionalPresentVar!=null) {
                            result.put(optionalPresentVar, false);
                            valueUpdater = null;
                        }
                        optionalDepth++;
                        cr = doCall();
                        if (cr.isPresent()) {
                            continue;
                        }
                    }
                    return cr;
                }

                isEndOfOptional = t.endsWith("]");

                if (isEndOfOptional) {
                    t = t.substring(0, t.length() - 1);
                    if (t.isEmpty()) continue;
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core/src/main/java/org/apache/brooklyn/core/workflow/ShorthandProcessorQst.java [125:227]:
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                } else {
                    // shouldn't come here
                    return Maybe.absent("Input has trailing characters after template is matched: '" + inputRemaining + "'");
                }
            }
            return Maybe.of(result);
        }

        protected Maybe<Object> doCall() {
            boolean isEndOfOptional = false;
            outer: while (true) {
                if (isEndOfOptional) {
                    if (optionalDepth <= 0) {
                        throw new IllegalStateException("Unexpected optional block closure");
                    }
                    optionalDepth--;
                    if (optionalSkippingInput>0) {
                        // we were in a block where we skipped something optional because it couldn't be matched; outer parser is now canonical,
                        // and should stop skipping
                        return Maybe.of(true);
                    }
                    isEndOfOptional = false;
                }

                if (templateTokens.isEmpty()) {
                    if (Strings.isNonBlank(inputRemaining) && valueUpdater==null) {
                        return Maybe.absent("Input has trailing characters after template is matched: '" + inputRemaining + "'");
                    }
                    if (optionalDepth>0)
                        return Maybe.absent("Mismatched optional marker in template");
                    return Maybe.of(true);
                }
                String t = templateTokens.remove(0);

                if (t.startsWith("[")) {
                    t = t.substring(1);
                    if (!t.isEmpty()) {
                        templateTokens.add(0, t);
                    }
                    String optionalPresentVar = null;
                    if (!templateTokens.isEmpty() && templateTokens.get(0).startsWith("?")) {
                        String v = templateTokens.remove(0);
                        if (v.startsWith("?${") && v.endsWith("}")) {
                            optionalPresentVar = v.substring(3, v.length() - 1);
                        } else {
                            throw new IllegalStateException("? after [ should indicate optional presence variable using syntax '?${var}', not '"+v+"'");
                        }
                    }
                    Maybe<Object> cr;
                    if (optionalSkippingInput<=0) {
                        // make a deep copy so that valueUpdater writes get replayed
                        Map<String, Object> backupResult = (Map) CollectionMerger.builder().deep(true).build().merge(MutableMap.of(), result);
                        Consumer<String> backupValueUpdater = valueUpdater;
                        String backupInputRemaining = inputRemaining;
                        List<String> backupTemplateTokens = MutableList.copyOf(templateTokens);
                        int oldDepth = optionalDepth;
                        int oldSkippingDepth = optionalSkippingInput;

                        optionalDepth++;
                        cr = doCall();
                        if (cr.isPresent()) {
                            // succeeded
                            if (optionalPresentVar!=null) result.put(optionalPresentVar, true);
                            continue;

                        } else {
                            // restore
                            result = backupResult;
                            valueUpdater = backupValueUpdater;
                            if (optionalPresentVar!=null) result.put(optionalPresentVar, false);
                            inputRemaining = backupInputRemaining;
                            templateTokens.clear();
                            templateTokens.addAll(backupTemplateTokens);
                            optionalDepth = oldDepth;
                            optionalSkippingInput = oldSkippingDepth;

                            optionalSkippingInput++;
                            optionalDepth++;
                            cr = doCall();
                            if (cr.isPresent()) {
                                optionalSkippingInput--;
                                continue;
                            }
                        }
                    } else {
                        if (optionalPresentVar!=null) {
                            result.put(optionalPresentVar, false);
                            valueUpdater = null;
                        }
                        optionalDepth++;
                        cr = doCall();
                        if (cr.isPresent()) {
                            continue;
                        }
                    }
                    return cr;
                }

                isEndOfOptional = t.endsWith("]");

                if (isEndOfOptional) {
                    t = t.substring(0, t.length() - 1);
                    if (t.isEmpty()) continue;
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