diff --git a/lib/Minilisp.pm b/lib/Minilisp.pm
index bcce925..5f0a6ea 100644
--- a/lib/Minilisp.pm
+++ b/lib/Minilisp.pm
@@ -271,6 +271,13 @@ my %stdctx = (
       }
     return $v;
   },
+  'mod' => sub {
+    my ($number, $divisor) = @_;
+    $number += $divisor while ( $number < 0 );
+    $number -= $divisor while ( $number >= $divisor );
+
+    return $number;
+  },
 
   # String comparison
   'string=' => sub { my ($a, $b) = @_; return to_lisp_bool($a eq $b); },