in hadoop-api-shim/src/main/java/org/apache/hadoop/fs/shim/impl/AbstractFSBuilderImpl.java [136:141]
public B opt(@Nonnull final String key, @Nonnull final String value) {
mandatoryKeys.remove(key);
optionalKeys.add(key);
options.set(key, value);
return getThisBuilder();
}