The .classify method

    %addrbook.classify({ .value.substr(0,1).uc })

• .classify
• applies the block to each element
• returns a list of pairs where
• keys are the values returned from applying the block
• values are arrays of values that produced that key

• In this case, .classify is being applied to %addrbook, which acts like a list of "email => name" Pairs.

• The { .value.substr(0,1).uc } block is applied to each Pair, returning the uppercase value of the first letter of the name.