code: void addMixin( Rule mixin, Map<String, Expression> parameters, Map<String, Expression> variables ) {
          int idx = state.stackIdx++;
          Scope scope;
          if( state.stack.size() <= idx ) {
              scope = new Scope();
              state.stack.add( scope );
          } else {
              scope = state.stack.get( idx );
              scope.returns.clear();
          }
          scope.mixin = mixin;
          scope.parameters = parameters;
          scope.variables = variables;
      }
docs: Add the scope of a mixin to the stack.
  @param mixin the mixin
  @param parameters the calling parameters
  @param variables the variables of the mixin