-- fld fn xs init: left fold with an explicit initial accumulator. The fn
-- takes (acc, item) and returns the new acc. Initial value carries the
-- starting state. Returns the final acc, not a list.
-- Signature: fld fn:F a b a xs:L b a init:a > a
add a:n b:n>n;+a b
main xs:L n>n;fld add xs 0
-- run: main [1,2,3,4,5]
-- out: 15