tiny fixes

src/ast.rs

@@ -95,7 +95,7 @@ impl ASTArena {
             if ch == '\n' {
                 if i < span.start as usize {
                     line_number += 1;
-                    start_of_line = i;
+                    start_of_line = i + 1;
                 }
                 if i >= span.end as usize && end_of_line.is_none() {
                     end_of_line = Some(i);

src/builtins.rs

@@ -4,9 +4,10 @@ use crate::interp::*;
 
 fn arity_error(func: &str, expected: usize, actual: &[ExprRef]) -> Result<Value, MatzoError> {
     let msg = format!(
-        "`{}`: expected {} arguments, got {}",
+        "`{}`: expected {} argument{}, got {}",
         func,
         expected,
+        if expected == 1 { "" } else { "s" },
         actual.len()
     );
     if actual.is_empty() {