interpretStmts (h:t) = do exp <- iStmt h
                          case exp of
                            Null -> interpretStmts t;
                            otherwise -> return exp;
interpretStmts (h:t) = do exp <- iStmt h
                          case exp of
                            Null -> interpretStmts t;
                            otherwise -> return exp;
interpretStmts (h:t) = do exp <- iStmt h
                          case exp of
                            Null -> interpretStmts t;
                            otherwise -> return exp;