diff --git a/k&r/04-funcs-and-prog-struct/04-calc.c b/k&r/04-funcs-and-prog-struct/04-calc.c
index b564b2f..895bb98 100644
--- a/k&r/04-funcs-and-prog-struct/04-calc.c
+++ b/k&r/04-funcs-and-prog-struct/04-calc.c
@@ -4,31 +4,55 @@
 
 
 #define printerr(format, ...) printf("error: " format "\n", ##__VA_ARGS__)
+int stack_len(void);
 void push(double x);
 double pop(void);
-int stack_len(void);
+double peek(void);
+void clear(void);
+
+/* --------- PROMPT FORMATS */
+#define PROMPT_F     ">> "
+#define POP_PRINT_F  "pop -> %.8g\n"
+#define PEEK_PRINT_F "peek -> %.8g\n"
+#define EMPTY_F      "empty\n"
 
 /* --------- ERROR MESSAGES */
 #define ZERODIV_ERR "zero divisor"
+#define FULL_STACK_ERR  "stack full, can't push %g"
+#define EMPTY_STACK_ERR "stack empty" 
+
+/* --------- CALCULATOR FUNCTIONS TYPES */
+/* command application */
+#define COMMAND(CMD) CMD()
+
+/* commutative operator */
+#define COMM_OP(OP) push(pop() OP pop()) 
+/* non-commutative operator */
+#define NONC_OP(OP, T, ...) \
+    do { T op2 = pop(); __VA_ARGS__ push(((T) pop()) OP op2); } while (0)
+/* nonzero second operand */
+#define NONZ_SND_OP if (!op2) printerr(ZERODIV_ERR); else
+
 
 
 /* --------- CALCULATOR FUNCTIONS */
-/* commutative operator application */
-#define COMM_OP(OP) push(pop() OP pop()) 
-/* non-commutative operator application */
-#define NONC_OP(OP, T, ...) \
-    do { T op2 = pop(); __VA_ARGS__ push(((T) pop()) OP op2); } while (0)
-/* nonzero second operand operator application */
-#define NONZ_OP(OP, T) \
-    NONC_OP(OP, T, if (!op2) printerr(ZERODIV_ERR); else)
-
 #define FUNCTIONS \
     X(+, COMM_OP) \
     X(*, COMM_OP) \
     X(-, NONC_OP, double) \
-    X(/, NONZ_OP, double) \
-    X(%, NONZ_OP, int) 
+    X(/, NONC_OP, double, NONZ_SND_OP) \
+    X(%, NONC_OP, int, NONZ_SND_OP) \
+    X(print, COMMAND) \
+    X(dup, COMMAND) \
+    X(swap, COMMAND) \
+    X(clear, COMMAND)
 
+/* --------- COMMAND DEFINITIONS */ 
+
+#define print() printf(PEEK_PRINT_F, peek())
+#define dup() push(peek())
+#define swap() \
+    do { double fst, snd; snd = pop(), fst = pop(); push(snd), push(fst); } while (0)
 
 
 #define MAXTOKEN 100
@@ -40,11 +64,16 @@ int main()
     char token[MAXTOKEN + 1];
     double op2;
     int t_len;
-
+    
+    printf(PROMPT_F);
     while (t_len = gettoken(token, MAXTOKEN)) {
-        if (t_len < 0) 
-            printf("\t%g\n",  pop());
-        else
+        if (t_len < 0) { /* newline token */
+            if (stack_len())
+                printf(POP_PRINT_F,  pop());
+            else
+                printf(EMPTY_F);
+            printf(PROMPT_F);
+        } else
     #define X(F, APPLY, ...) \
         if (!strcmp(#F, token)) \
             APPLY(F, ##__VA_ARGS__); \
@@ -53,7 +82,10 @@ int main()
     #undef X
         push(atof(token));
     }
-    printf("%g\n", pop());
+    if (!stack_len())
+        putchar('\n');
+    while (stack_len())
+        printf(POP_PRINT_F, pop());
 
     return 0;
 }
@@ -98,14 +130,14 @@ int stack_len()
     return sp;
 }
 
+
 /* push: push f onto value stack */
 void push(double f)
 {
     if (sp < MAXVAL)
         val[sp++] = f;
     else
-        printf("error: stack full, can't push %g\n", f);
-
+        printerr(FULL_STACK_ERR, f);
 }
 
 /* pop: pop and return top value from stack */
@@ -113,8 +145,20 @@ double pop(void)
 {
     if (sp)
         return val[--sp];
-    printf("error: stack empty\n");
+    printerr(EMPTY_STACK_ERR);
+    return 0;
+}
+
+/* peek: return top value from stack, leave stack unchanged */
+double peek(void)
+{
+    if (sp)
+        return val[sp-1];
+    printerr(EMPTY_STACK_ERR);
     return 0.0;
 }
 
-
+void clear(void)
+{
+    sp = 0;
+}