src/main/java/org/apache/commons/text/StrBuilder.java [431:459]:
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        if (buf == null) {
            return appendNull();
        }
        if (buf.hasArray()) {
            final int totalLength = buf.remaining();
            if (startIndex < 0 || startIndex > totalLength) {
                throw new StringIndexOutOfBoundsException("startIndex must be valid");
            }
            if (length < 0 || startIndex + length > totalLength) {
                throw new StringIndexOutOfBoundsException("length must be valid");
            }
            final int len = length();
            ensureCapacity(len + length);
            System.arraycopy(buf.array(), buf.arrayOffset() + buf.position() + startIndex, buffer, len, length);
            size += length;
        } else {
            append(buf.toString(), startIndex, length);
        }
        return this;
    }

    /**
     * Appends a CharSequence to this string builder.
     * Appending null will call {@link #appendNull()}.
     *
     * @param seq  the CharSequence to append
     * @return this, to enable chaining
     */
    @Override
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src/main/java/org/apache/commons/text/TextStringBuilder.java [487:514]:
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        if (buf == null) {
            return appendNull();
        }
        if (buf.hasArray()) {
            final int totalLength = buf.remaining();
            if (startIndex < 0 || startIndex > totalLength) {
                throw new StringIndexOutOfBoundsException("startIndex must be valid");
            }
            if (length < 0 || startIndex + length > totalLength) {
                throw new StringIndexOutOfBoundsException("length must be valid");
            }
            final int len = length();
            ensureCapacity(len + length);
            System.arraycopy(buf.array(), buf.arrayOffset() + buf.position() + startIndex, buffer, len, length);
            size += length;
        } else {
            append(buf.toString(), startIndex, length);
        }
        return this;
    }

    /**
     * Appends a CharSequence to this string builder. Appending null will call {@link #appendNull()}.
     *
     * @param seq the CharSequence to append
     * @return this, to enable chaining
     */
    @Override
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