code: public void clear(final String propertyName) {
          Preconditions.checkNotNull(propertyName);
          if (properties.containsKey(propertyName)) {
              properties.put(propertyName, null);
          } else if (references.containsKey(propertyName)) {
              references.put(propertyName, null);
          }
      }
docs: Clear the values of a property or reference, setting it to null.
  
  This operation can be useful when removing properties using "merge"
  operation.
  
  @param propertyName of the property as defined by the bean's schema.