code: public boolean isTrue(final URI property) throws IllegalStateException,
              IllegalArgumentException {
          final Boolean value = getUnique(property, Boolean.class);
          return value != null && value.booleanValue();
      }
docs: Determines whether the property specified has been set to true. The method fails if the
  property has multiple values or has a non-boolean type; if this behaviour is not desired,
  use {@link #getUnique(URI, Class, Object)} specifying {@code Boolean.class} as the class
  and an appropriate default value to be returned in case of failure.
  
  @param property
  the property to read
  @return true if the property is set to true; false if the property has no value or has been
  set to false
  @throws IllegalStateException
  in case the property has multiple values
  @throws IllegalArgumentException
  in case the property value is not of boolean type