code: function(atom1, atom2) {
          if (this.isVariable(atom1) && this.isVariable(atom2) ) {
              return true;
          } else if(atom1 == atom2) {
              return true;
          } else {
              return false;
          }
      }
docs: Return true if the two atoms are either both variables, or
  identical URIs.
  @returns {boolean}