procedure add(s1,s2,carry)
local size, sum
/carry := 0 # default carry
if *s1 > *s2 then s1 :=: s2
size := *s2
if size <= segsize then return s1 + s2 + carry
s1 := right(s1,size,"0")
sum := right(s1,segsize) + right(s2,segsize) + carry
return add(left(s1,size - segsize),
left(s2,size - segsize),sum / base) ||
right(sum % base,segsize,"0")
end