Why is PIPPIN grumbling or singing?

When you are keying in a program PIPPIN will grumble at you if:-

*    You have not entered any numbers (red keys) after an instruction other than and you try to enter another instruction.

*    The memory is full. You can only enter 100 separate commands in one program.

*    You have pressed when the memory is already empty.

*    You have pressed 0 after a command that needs a number.

*    You have pressed a number before entering a command.

*    You have tried to enter too many numbers after a command. You can enter up to three numbers after all the commands except which needs just one number and which needs no number.

*    You have tried to nest 's more than three deep.

*    You have tried to put in an without a matching .

*    You have left PIPPIN on for more than three minutes without pressing any buttons.

When you press or , PIPPIN will grumble at you if:-

*    You have entered a command without an amount.

*    You have entered a without a matching .

*    PIPPIN has been stopped from running by running into something.

*    You pressed a key to stop PIPPIN.

*    If PIPPIN's battery is flat.

PIPPIN will sing when:-

*    You turn PIPPIN on.

*    Your program has run without any mistakes in it.