add append function

src/str.rs

@@ -21,6 +21,8 @@ use crate::segment::{Ctr, Ast, circuit, ast_as_string};
 use std::rc::Rc;
 use std::cell::RefCell;
 
+// Current primitive is to use a get_NNNNN function defined for each library function which returns a not-previously-owned
+// copy of the Function struct.
 pub fn get_echo() -> Function {
     return Function{
         name: String::from("echo"),
@@ -44,8 +46,7 @@ pub fn get_echo() -> Function {
                     string.push_str("\n");
                     return true;
                 }) {
-                    // TODO: use custom logger
-                    println!("Echo failure!")
+                    eprintln!("circuit loop in echo should not have returned false")
                 }
                 return Ctr::String(string);
             }