! increment the value, or if the key does not exist, set the value to 1
: increment-val-for-key ( key assoc -- )
    2dup
    at* 
    [ 1 + ] [ drop 1 ] if
    -rot        ! have key assoc val, need val key assoc for input to set-at
    set-at ;

! count the number of occurrences of each character in str
: count-occurrences ( str -- assoc )
    V{ } dup
    -rot
    [ increment-val-for-key ] curry each ;