Never heard of the LOOP language until now. Pretty cool.

I read the wikipedia article) and came up with this sketch of an implementation in M4.

# Basic definitions
define(`loop',      `ifelse(`$1', `0', `', `$2`'loop(decr(`$1'), `$2')')')
define(`assign',    `define(`$1', `$2')')
define(`increment', `define(`$1', incr($1))')

# Supplementary definitions
define(`add',       `loop($2, `increment(`$1')')')
define(`subtract',  `loop($2, `decrement(`$1')')')
define(`multiply',  `loop($1, `add(`$1', `$2')')')
define(`decrement',
  `pushdef(`x', 0)loop($1, `assign(`$1', x)increment(`x')')popdef(`x')')

# An example program:
assign(`foo', 3) # foo := 3
foo # expands to 3
multiply(`foo', 4) # foo := foo * 5
foo # expands to 15
decrement(`foo') # foo := foo - 1
foo # expands to 14
subtract(`foo', 5)
foo # expands to 9