warning about E

src/parsing.rs

@@ -120,7 +120,16 @@ pub fn test_func(function_string: &str) -> Option<String> {
                     .collect::<Vec<String>>();
 
                 return match var_names_not_x.len() {
-                    1 => Some(format!("Error: invalid variable: {}", var_names_not_x[0])),
+                    1 => {
+                        let var_name = &var_names_not_x[0];
+                        if var_name == "e" {
+                            Some(String::from(
+                                "If trying to use Euler's number, please use an uppercase E",
+                            ))
+                        } else {
+                            Some(format!("Error: invalid variable: {}", var_name))
+                        }
+                    }
                     _ => Some(format!("Error: invalid variables: {:?}", var_names_not_x)),
                 };
             }