/* Routines to do substitutions, replacing one pattern with another */
/* eg, replace an expression of the from x-x by 0 or x+x by 2*x */

typedef struct {
    pattern *pat; /* the pattern */
    value rep; /* What to replace it with */
} subst;

subst *make_sub(char *, char *);
void free_sub(subst *);
value substitute(subst *, value); /* creates a new expression with the substitution done (if possible) */