Update tut1.txt

doc/tut1.txt

@@ -758,9 +758,13 @@ However, this cannot be done for mutually recursive procedures:
   proc even(n: int): bool
 
   proc odd(n: int): bool =
+    if n == 0: 
+      true
     n == 1 or even(n-1)
 
   proc even(n: int): bool =
+    if n == 1: 
+      true
     n == 0 or odd(n-1)
 
 Here ``odd`` depends on ``even`` and vice versa. Thus ``even`` needs to be