substitutes all occurrences based in the it object
function(input, it) {
      var output = input;
      var expr;
      while((expr = this.getExpr(output)) !== null) {
        var evalsTo = $parse(expr.expr)({it: it});
        output = output.substring(0, expr.start) + evalsTo + output.substring(expr.stop + 1);
      }
      return output;
    }