code: function (cmd, parentCmd, pos) {
      var S  = 'Somme';
      var D  = 'Diff';
      var P  = 'Produit';
      var Q  = 'Quotient';
      var O  = 'Opposé';
      var I  = 'Inverse';
      var C  = 'Carré';
      var Pu = 'Puissance';
      var R  = 'Racine';
      if (belongsTo(parentCmd, [C,Pu]) && pos == 0) {
          return belongsTo(cmd, [S,D,O,P,Q,I,C,Pu]);
      }
      if (parentCmd == P) {
          return belongsTo(cmd, [S,D,O]);
      }
      if ((parentCmd == O) || (parentCmd == D && pos == 1)) {
          return belongsTo(cmd, [S,D,O]);
      }
      if (parentCmd == S && cmd == O && pos == 1) {
          return true;
      }
      return false;
  }
docs: @summary Tests if the sub-expr should be surrounded with parens.
  
  @param cmd The current type of expr, aka command
  @param parentCmd The parent command
  @pos Position in the list of args, starting with 0
  @return a boolean