code: function(elem, mapping) {
          if(Logics.isBNode(elem)) {
              return Logics.skolemize(mapping.__facts__, elem);
          } else if(Logics.isVariable(elem)) {
              if (mapping[elem] !== undefined) {
                  return mapping[elem]
              }
          }
          return elem;
      }
docs: Substitutes an element given the mapping.
  @param elem
  @param mapping
  @returns {*}