C/in keyword



element C container
evaluates to true/1B if
element
is contained in the
container
.
element !C container
evaluates to true/1B if
element
is not contained in the
container
.

Note that this code:
I source[i] C ‘a’..‘z’ | source[i] C ‘A’..‘Z’ | source[i] C ‘0’..‘9’ | source[i] == ‘_’
   ...
can be shortened to:
I source[i] C (‘a’..‘z’, ‘A’..‘Z’, ‘0’..‘9’, ‘_’)
   ...