represents the pattern object obj, assigned the name s.
- The name s must be a symbol.
- The object obj can be any pattern object.
- When a transformation rule is used, any occurrence of s on the right‐hand side is replaced by whatever expression it matched on the left‐hand side.
- The operator : has a comparatively low precedence. The expression x:_+_ is thus interpreted as x:(_+_), not (x:_)+_.
- The form s_ is equivalent to s:_. Similarly, s_h is equivalent to s:_h, s__ to s:__, and so on.
Introduced in 1988