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 is thus interpreted as , not .
- The form s_ is equivalent to s:_. Similarly, s_h is equivalent to , s__ to s:__, and so on.
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