This is a golfed-down version of the Fac() - Calculate Factorial of a natural number UDF.
The function basically multiplies the number n with the factorial of the previous number n-1 with the boundary condition of 1!=1
Code:

function fac($)
  if $>1
    $fac=cdbl($)*fac($-1)
  else
    $fac=1
  endif
endfunction
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