group the elements of this sequence by a key of type and applies function f to all elements

example: group characters by category and add underscores around each character

values array codepoint := ["A", "1", "b", "?", "X", "4"]

classify (c codepoint) String => c.is_ascii_letter ? "letter" : c.is_ascii_digit ? "digit" : "other"

values.group_map String String classify (x->"_{x}_")

=> {(digit => [_1_, _4_]), (letter => [_A_, _b_, _X_]), (other => [_?_])}