-- appends two lists
append  :: [a] -> [a] -> [a]
append [][]        = []
append [][c]       = [c]
append [][c,d]     = [c,d]
append [] [a,b,c]  = [a,b,c]
append [][a,b,c,d] = [a,b,c,d]
append [a][]       = [a]
append [a][c]      = [a,c]
append [a,b][]     = [a,b]
append [a][c,d]    = [a,c,d]
append [a,b][d]    = [a,b,d]
append [a,c,d][]   = [a,c,d]
append [a,b][c,d]  = [a,b,c,d]
append [a,b,c][d]  = [a,b,c,d]
append [a,b,c,d][] = [a,b,c,d]