Add some documentation, and clean up EvalError a bit.

src/eval/primop.rs

@@ -68,7 +68,11 @@ impl Value {
     /// Calculate the result of running the given primitive on the given arguments.
     /// 
     /// This can cause errors in a whole mess of ways, so be careful about your
-    /// inputs.
+    /// inputs. For example, addition only works when the two values have the exact
+    /// same type, so expect an error if you try to do so. In addition, this
+    /// implementation catches and raises an error on overflow or underflow, so
+    /// its worth being careful to make sure that your inputs won't cause either
+    /// condition.
     pub fn calculate(operation: &str, values: Vec<Value>) -> Result<Value, PrimOpError> {
         if values.len() == 2 {
             Value::binary_op(operation, &values[0], &values[1])