code: public boolean add(ExtendedPropositionDefinition def) {
          if (def != null && this.defs.add(def)) {
              this.defsAsListOutdated = true;
              recalculateChildren();
              return true;
          } else {
              return false;
          }
      }
docs: Adds an extended proposition definition.
  
  @param def
  an {@link ExtendedPropositionDefinition}.
  @return <code>true</code> if successfully added, <code>false</code>
  otherwise (e.g., the {@link ExtendedPropositionDefinition} is
  <code>null</code> or is already part of this abstraction
  definition.