code: String getTarget(Object source) {
          if (this.mappings != null) {
              return this.mappings.getTarget(source);
          } else {
              return null;
          }
      }
docs: Returns the proposition id corresponding to a specified value in this
  column spec's column.
  
  @param source
  the value.
  @return a proposition id {@link String} or <code>null</code> if a value
  is specified that is not in this column spec's mappings from
  proposition ids to values ({@link #getMappings() }
  ).