code: function installVectorArithmetic() {
          // plugin.install can be used several times per Less session
          // (e.g. Grunt compiling multiple files per task), hence ensure
          // we hook to the real built-in instead of an earlier hook:
          var id = '__pluginListsArithm',
              p  = Operation.prototype;
          builtinArithmEval = p.eval;
          if (builtinArithmEval[id])
              builtinArithmEval = builtinArithmEval[id].builtin;
          p.eval     = arithmEval;
          p.eval[id] = {builtin: builtinArithmEval};
          
      }
docs: ........................................................