Y must be numeric excluding negative integers.  R is numeric.  R is the product of the first Y integers for positive integer values of Y. In general, !Y is gamma Y+1.

Examples

      !1 2 3 4 5
1 2 6 24 120
 
      !¯1.5 0 1.5 3.3
¯3.544907702 1 1.329340388 8.85534336
 
      !0j1 1j2
0.4980156681J¯0.1549498283 0.1122942423J0.3236128855