procedure main()
write("a dot b := ",dotproduct([1, 3, -5],[4, -2, -1]))
end

procedure dotproduct(a,b)   #: return dot product of vectors a & b or error
if *a ~= *b & type(a) == type(b) == "list" then runerr(205,a) # invalid value
every (dp := 0) +:= a[i := 1 to *a] * b[i]
return dp
end