Resolve "Cleanup parser"

Closes #41 (closed)

eraser-base/src/main/jastadd/Expression.jrag

@@ -30,4 +30,15 @@ aspect Expression {
   eq Designator.eval() = getItem().getStateAsDouble();
   eq ParenthesizedNumberExpression.eval() = getOperand().eval();
   eq NumberLiteralExpression.eval() = getValue();
+
+  // is-X
+  syn boolean Expression.isLogicalExpression() = false;
+  eq LogicalExpression.isLogicalExpression() = true;
+  syn boolean Expression.isNumberExpression() = false;
+  eq NumberExpression.isNumberExpression() = true;
+  // as-X
+  syn LogicalExpression Expression.asLogicalExpression() = null;
+  eq LogicalExpression.asLogicalExpression() = this;
+  syn NumberExpression Expression.asNumberExpression() = null;
+  eq NumberExpression.asNumberExpression() = this;
 }