8D. Arithmetic

d0=: [: >: #:

Add 1 to cyclic counter y with upper limit x

d1=: +//.

Sum items of y corresponding to keys x

d2=: +//. {~ 'abcde'"_ i. ~.@[ 

Sum y according to keys x from key set 'abcde'. (substitute your ordered keyset for 'abcde')

d3=: +.

Greatest common divisor of x and y

m4=: >:@i.@(>./) |/ ]

1 thru max y residues of y

m5=: 0: = m4

Locate positive integers <: y dividing y

d6=: +"1

Add x to each row of y

d7=: +

Add list x to each column of table y

d8=: +

Add list y to each column of table x

d9=: *.

Least common multiple of x and y