This structure defines a law, with a spine, in conjunction with the
UF_MODL_parm structure.

Note for all character string values in the structure, if the character
string is in expression format (e.g., p5=25, p5+p7), and the
expression does not exist, an expression is created and the law is
associated to that expression. If the expression does exist, then the
existing expression is edited, and the law is associated to that
expression. If the character string contains an expression name
(e.g., p5) and the expression exists, the law is associated to that
expression, if the expression does not exist an error occurs. If
the character string is a numeric value (e.g., 25), the law is not
associated to an expression.