diff --git a/src/conspage.c b/src/conspage.c
index e016c86..0b13baf 100644
--- a/src/conspage.c
+++ b/src/conspage.c
@@ -73,9 +73,11 @@ void make_cons_page(  ) {
                         strncpy( &cell->tag.bytes[0], TRUETAG, TAGLENGTH );
                         cell->count = MAXREFERENCE;
                         cell->payload.free.car = ( struct cons_pointer ) {
-                        0, 1};
+                            0, 1
+                        };
                         cell->payload.free.cdr = ( struct cons_pointer ) {
-                        0, 1};
+                            0, 1
+                        };
                         fwprintf( stderr, L"Allocated special cell T\n" );
                         break;
                 }
@@ -110,7 +112,8 @@ void dump_pages( FILE * output ) {
 
         for ( int j = 0; j < CONSPAGESIZE; j++ ) {
             dump_object( output, ( struct cons_pointer ) {
-                         i, j} );
+                         i, j
+                         } );
         }
     }
 }
diff --git a/src/consspaceobject.c b/src/consspaceobject.c
index defc56f..30bfa83 100644
--- a/src/consspaceobject.c
+++ b/src/consspaceobject.c
@@ -63,7 +63,7 @@ void dec_ref( struct cons_pointer pointer ) {
     }
 }
 
-void dump_string_cell( FILE * output, wchar_t * prefix,
+void dump_string_cell( FILE * output, wchar_t *prefix,
                        struct cons_pointer pointer ) {
     struct cons_space_object cell = pointer2cell( pointer );
     if ( cell.payload.string.character == 0 ) {
@@ -129,10 +129,10 @@ void dump_object( FILE * output, struct cons_pointer pointer ) {
                       cell.payload.integer.value, cell.count );
             break;
         case LAMBDATV:
-            fwprintf( output, L"Lambda cell; args: " );
+            fwprintf( output, L"\t\tLambda cell; args: " );
             print( output, cell.payload.lambda.args );
             fwprintf( output, L";\n\t\t\tbody: " );
-            print( output, cell.payload.lambda.args );
+            print( output, cell.payload.lambda.body );
             break;
         case READTV:
             fwprintf( output, L"\t\tInput stream\n" );
diff --git a/src/init.c b/src/init.c
index e782e9a..9c217c8 100644
--- a/src/init.c
+++ b/src/init.c
@@ -48,12 +48,14 @@ int main( int argc, char *argv[] ) {
 
     while ( ( option = getopt( argc, argv, "pd" ) ) != -1 ) {
         switch ( option ) {
+            case 'c':
+                print_use_colours = true;
+                break;
             case 'd':
                 dump_at_end = true;
                 break;
             case 'p':
                 show_prompt = true;
-                print_use_colours = true;
                 break;
             default:
                 fwprintf( stderr, L"Unexpected option %c\n", option );
@@ -106,6 +108,7 @@ int main( int argc, char *argv[] ) {
     bind_special( "cond", &lisp_cond );
     bind_special( "lambda", &lisp_lambda );
     bind_special( "quote", &lisp_quote );
+    bind_special( "set!", &lisp_set_shriek );
 
 
     /* bind the oblist last, at this stage. Something clever needs to be done
diff --git a/src/lispops.c b/src/lispops.c
index b2f7800..945c412 100644
--- a/src/lispops.c
+++ b/src/lispops.c
@@ -128,11 +128,10 @@ eval_lambda( struct cons_space_object cell, struct stack_frame *frame,
     struct cons_pointer new_env = env;
     struct cons_pointer args = cell.payload.lambda.args;
     struct cons_pointer body = cell.payload.lambda.body;
-    struct cons_pointer vals = frame->arg[0];
 
-    while ( consp( args ) && consp( vals ) ) {
+    for ( int i = 0; i < args_in_frame && consp( args ); i++ ) {
         struct cons_pointer arg = c_car( args );
-        struct cons_pointer val = c_car( vals );
+        struct cons_pointer val = frame->arg[i];
         print( stderr, c_string_to_lisp_string( "\n\tBinding " ) );
         print( stderr, arg );
         print( stderr, c_string_to_lisp_string( " to " ) );
@@ -141,8 +140,6 @@ eval_lambda( struct cons_space_object cell, struct stack_frame *frame,
 
         new_env = make_cons( make_cons( arg, val ), new_env );
         args = c_cdr( args );
-        vals = c_cdr( vals );
-
     }
 
     while ( !nilp( body ) ) {
@@ -194,6 +191,8 @@ c_apply( struct stack_frame *frame, struct cons_pointer env ) {
             {
                 struct stack_frame *next =
                     make_stack_frame( frame, args, env );
+                fputws( L"Stack frame for lambda\n", stderr );
+                dump_frame( stderr, next );
                 result = eval_lambda( fn_cell, next, env );
                 free_stack_frame( next );
             }
@@ -283,6 +282,7 @@ lisp_eval( struct stack_frame *frame, struct cons_pointer env ) {
                     result = lisp_throw( message, frame );
                 } else {
                     result = c_assoc( canonical, env );
+                    inc_ref( result );
                 }
             }
             break;
@@ -341,6 +341,33 @@ lisp_quote( struct stack_frame *frame, struct cons_pointer env ) {
     return frame->arg[0];
 }
 
+/**
+ * (set! symbol value)
+ * (set! symbol value namespace)
+ *
+ * Special form.
+ * `namespace` defaults to the oblist.
+ * Binds `symbol` to `value` in the namespace, altering the namespace in so doing.
+ */
+struct cons_pointer
+lisp_set_shriek( struct stack_frame *frame, struct cons_pointer env ) {
+    struct cons_pointer result = NIL;
+    struct cons_pointer namespace =
+        nilp( frame->arg[2] ) ? oblist : frame->arg[2];
+
+    if ( symbolp( frame->arg[0] ) ) {
+        deep_bind( frame->arg[0], eval_form( frame, frame->arg[1], env ) );
+        result = frame->arg[1];
+    } else {
+        result =
+            make_exception( c_string_to_lisp_string
+                            ( "The first argument to `set!` is not a symbol" ),
+                            frame );
+    }
+
+    return result;
+}
+
 /**
  * (cons a b)
  *
diff --git a/src/lispops.h b/src/lispops.h
index fac1ec0..dbbf55a 100644
--- a/src/lispops.h
+++ b/src/lispops.h
@@ -31,13 +31,17 @@
 struct cons_pointer c_type( struct cons_pointer pointer );
 
 /*
- * special forms 
+ * special forms
  */
 struct cons_pointer lisp_eval( struct stack_frame *frame,
                                struct cons_pointer env );
 struct cons_pointer lisp_apply( struct stack_frame *frame,
                                 struct cons_pointer env );
-                                /**
+
+struct cons_pointer
+lisp_set_shriek( struct stack_frame *frame, struct cons_pointer env );
+
+/**
  * The Lisp interpreter.
  *
  * @param frame the stack frame in which the expression is to be interpreted;
@@ -51,7 +55,7 @@ struct cons_pointer lisp_quote( struct stack_frame *frame,
                                 struct cons_pointer env );
 
 /*
- * functions 
+ * functions
  */
 struct cons_pointer lisp_cons( struct stack_frame *frame,
                                struct cons_pointer env );
@@ -80,22 +84,22 @@ lisp_type( struct stack_frame *frame, struct cons_pointer env );
 
 
 /**
- * Function; evaluate the forms which are listed in my single argument 
+ * Function; evaluate the forms which are listed in my single argument
  * sequentially and return the value of the last. This function is called 'do'
  * in some dialects of Lisp.
- * 
+ *
  * @param frame My stack frame.
  * @param env My environment (ignored).
- * @return the value of the last form on the sequence which is my single 
+ * @return the value of the last form on the sequence which is my single
  * argument.
  */
 struct cons_pointer
 lisp_progn( struct stack_frame *frame, struct cons_pointer env );
 
 /**
- * Special form: conditional. Each arg is expected to be a list; if the first 
- * item in such a list evaluates to non-NIL, the remaining items in that list 
- * are evaluated in turn and the value of the last returned. If no arg (clause) 
+ * Special form: conditional. Each arg is expected to be a list; if the first
+ * item in such a list evaluates to non-NIL, the remaining items in that list
+ * are evaluated in turn and the value of the last returned. If no arg (clause)
  * has a first element which evaluates to non NIL, then NIL is returned.
  * @param frame My stack frame.
  * @param env My environment (ignored).
@@ -105,7 +109,7 @@ struct cons_pointer
 lisp_cond( struct stack_frame *frame, struct cons_pointer env );
 
 /*
- * neither, at this stage, really 
+ * neither, at this stage, really
  */
 struct cons_pointer lisp_throw( struct cons_pointer message,
                                 struct stack_frame *frame );
diff --git a/unit-tests/intepreter.sh b/unit-tests/intepreter.sh
new file mode 100644
index 0000000..9eb2a06
--- /dev/null
+++ b/unit-tests/intepreter.sh
@@ -0,0 +1,12 @@
+#!/bin/bash
+
+expected='6'
+actual=`echo "(apply '(lambda (x y z) (/ (* y z) x)) '(2 3 4))" | target/psse 2> /dev/null | head -2 | tail -1`
+
+if [ "${expected}" = "${actual}" ]
+then
+    echo "OK"
+else
+    echo "Fail: expected '${expected}', got '${actual}'"
+    exit 1
+fi