[WIP] Implement subroutine opcodes

Author: Peppece

eth/abc.py

@@ -1590,6 +1590,21 @@ def stack_dup(self, position: int) -> None:
         """
         ...
 
+    #
+    # Return Stack Managemement
+    #
+    @abstract_method
+    def rstack_push_int(self) -> Callable[[int], None]:
+        """
+        Push integer onto the return stack.
+        """
+    
+    @abstract_method
+    def rstack_pop1_int(self) -> Callable[[int], None]:
+        """
+        Pop integer off the return stack and return it.
+        """
+
     #
     # Computation result
     #