catching the register command

suppose if i had mapped "p" to do some operation
based on some variable, if that variable is not set
the "p" should perform it's normal operation. now
how can i catch the register contents or count
before "p", for ex user might have pressed "ap,
how do i catch "a in this case.

fun! NewP()
if var == 1
"do something
else
"normal p
endfun

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