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- `c'
 - 
Clears the stack, rendering it empty.
 - `d'
 - 
Duplicates the value on the top of the stack,
pushing another copy of it.
Thus, `4d*p' computes 4 squared and prints it.
 - `r'
 - 
Reverses the order of (swaps) the top two values on the stack.
(This command is a GNU extension.)
 
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