Documentation for infix ⊉ assembled from the following pages:

Language documentation: Operators §

From Operators

(Operators) infix ⊉ §

multi sub infix:<⊉>($a,$b --> Bool:D)

Not a superset of nor equal to operator.

Returns True if $a is not a superset of $b. Equivalent to !(>=).

say (1,2,3) ⊉ (2,3,1); # OUTPUT: «False␤» 
say (1,2,3) ⊉ (2,3);   # OUTPUT: «False␤» 
say 4 !(>=) (1,2,3);   # OUTPUT: «True␤» 

Language documentation: Sets, bags, and mixes §

From Sets, bags, and mixes

(Sets, bags, and mixes) infix ⊉ §

Returns True if $a is not a superset nor equal to $b, else False. More information, Wikipedia definition.