Fix #10, add test

lib/index.js

@@ -182,7 +182,7 @@ const parseExpression = expression => {
         stack.push(`-${cur}`);
 
         // 2 - 5 is also OK, pass
-      } else if (_.isNumber(beforeSign)) {
+      } else if (_.isNumber(beforeSign) || isVariable(beforeSign)) {
         stack.push(cur);
       } else {
         stack[past] += cur;
@@ -379,9 +379,11 @@ const fixFloat = number => {
   * @return {Boolean}
   *         true if variable, else false
   */
+
+const SPECIALS = '()[]{}'.split('');
 export const isVariable = a => {
   return typeof a === 'string' && !_.isNumber(a) &&
-         !operators[a] && a === a.toLowerCase();
+         !operators[a] && a === a.toLowerCase() && SPECIALS.indexOf(a) === -1;
 };
 
 export default Equation;