[Description] Marker interface for function accept one argument or two arguments. [Interface] 
Operator
F r e e R e f F u n c t i o n
F u n c t i o n
F u n c t i o n 0 A r g
F u n c t i o n 1 A r g
F u n c t i o n 2 A r g
F u n c t i o n 3 A r g
F u n c t i o n 4 A r g
O p e r a t o r
X Y N u m e r i c F u n c t i o n . A c c u m u l a t o r