The RNPL compiler must know which names are coordinates. It further groups these names into systems. Some example coordinate declarations are:
rect coordinates t,x,y,z sph coordinates t,r,theta,phi
First comes a coordinate system name, then the reserved word coordinates, and then a comma-separated list of names. Order is important, with the first coordinate in the list taken to be time. Any time coordinate can be repeated in other coordinate systems, but each spatial coordinate can be used only once.