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Status Inquiry
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`Z'
     Pops a value off the stack, calculates the number of digits it has
     (or number of characters, if it is a string) and pushes that
     number.

`X'
     Pops a value off the stack, calculates the number of fraction
     digits it has, and pushes that number.  For a string, the value
     pushed is 0.

`z'
     Pushes the current stack depth; the number of objects on the stack
     before the execution of the `z' command.