iStmt (SIf exp stm)  = do {
    bexp <- calcBool exp;
    if bexp ==  BTrue then
        iStmt stm;
    else
        return Null;
  }
iStmt (SIf exp stm)  = do {
    bexp <- calcBool exp;
    if bexp ==  BTrue then
        iStmt stm;
    else
        return Null;
  }
iStmt (SIf exp stm)  = do {
    bexp <- calcBool exp;
    if bexp ==  BTrue then
        iStmt stm;
    else
        return Null;
  }