Expressions are made up mainly of identifiers separated by operators.
` RNPL` defines the usual set of arithmetic operators (` +,-,*,/`)
along with exponentiation (` ^` or ` **`). The operators obey the usual
precedence rules.

Identifiers may be names of grid functions, coordinates, or parameters. In addition, grid functions may be supplied with temporal and spatial offsets (see section 1.7), and coordinates may be supplied with a spatial offset.

Expressions can also contain the well-known functions exp, ln, tan, sin, cos, and sqrt as well as derivative operators.

Here is an example of a complicated ` RNPL` expression:

a*b+(c*2.67/<0>phi[1] + d^2)/tan(theta) - r[-1]*D_(phi*3/a,r) + cos(3*eta)*D_(eta+D_(phi,r),r) + expand D_(a*b + c,r)

You'll notice the word ` expand` before the last derivative operator.
This tells ` RNPL` to symbolically expand the derivative before making
the operator substitution. Thus, the final term is equivalent to:

D_(a,r)*b + a*D_(b,r) + D_(c,r)

Thu Jun 1 09:34:30 CDT 1995