From The Free On-line Dictionary of Computing (19 January 2023):
sum
1. <theory> In domain theory, the sum A + B of two domains
contains all elements of both domains, modified to indicate
which part of the union they come from, plus a new bottom
element. There are two constructor functions associated with
the sum:
inA : A -> A+B inB : B -> A+B
inA(a) = (0,a) inB(b) = (1,b)
and a disassembly operation:
case d of isA(x) -> E1; isB(x) -> E2
This can be generalised to arbitrary numbers of domains.
See also smash sum, disjoint union.
2. <tool> A Unix utility to calculate a 16-bit checksum of
the data in a file. It also displays the size of the file,
either in kilobytes or in 512-byte blocks. The checksum may
differ on machines with 16-bit and 32-bit ints.
Unix manual page: sum(1).
(1995-03-16)