l::Int->(Int->Double)->Int->Double->Double
l n xs k x = product [(x-xs i)/(xs k-xs i)|i<-[0..(k-1)]++[(k+1)..n]]
l::Int->(Int->Double)->Int->Double->Double
l n xs k x = product [(x-xs i)/(xs k-xs i)|i<-[0..(k-1)]++[(k+1)..n]]
l n xs k x = product [(x-xs i)/(xs k-xs i)|i<-[0..(k-1)]++[(k+1)..n]]