refactor: use assert, fix: respect to express settings.views, close #16

src/syntax.js

@@ -1,8 +1,9 @@
 const operators = require('./operators.js');
 const lexical = require('./lexical.js');
+const assert = require('../src/util/assert.js');
 
 function evalExp(exp, scope) {
-    if (!scope) throw new Error('unable to evalExp: scope undefined');
+    assert(scope, 'unable to evalExp: scope undefined');
     var operatorREs = lexical.operators,
         match;
     for (var i = 0; i < operatorREs.length; i++) {