The functions LOOKUP BACKWARD, LOOKUP EXTRAPOLATE and LOOKUP FORWARD provide alternative interpolation and extrapolation techniques for using Lookups. If you are using these functions you must explicitly denote that. For example the equations:
look((0,0),(10,0),(20,55),(30,76),(40,50),(50,50))
y = look(x)
y2 = LOOKUP EXTRAPOLATE(look,x)
would make y and y2 the same for all input values. The only difference being that no warning messages would be issued for y2 when x is out of range.
