iStmt (SIfElse exp stm1 stm2) = do {
    bexp <- calcBool exp;
    if bexp == BTrue then
        iStmt stm1;
    else
        iStmt stm2;
  }
iStmt (SIfElse exp stm1 stm2) = do {
    bexp <- calcBool exp;
    if bexp == BTrue then
        iStmt stm1;
    else
        iStmt stm2;
  }
iStmt (SIfElse exp stm1 stm2) = do {
    bexp <- calcBool exp;
    if bexp == BTrue then
        iStmt stm1;
    else
        iStmt stm2;
  }