! — Factorial / Binomial!Y: Y!. Pervasive.
!5 ⍝ 120
!5x ⍝ 120x
X!Y: binomial coefficient, Y choose X. Pervasive.
2!5 ⍝ 10
2x!5x ⍝ 10x
Extends through Gamma to non-integral and complex arguments.
DOMAIN: factorial poles.