Dave Herman wrote:

It would be nice to have a syntactic form := defined as follows:

     p1 := p2

is defined by

     !p2 => p1

where p1 and p2 are matching patterns.

For example, the common pattern of binding a new variable to the result
of applying a strategy:

     MyRule: x -> y
       where !<foo>(x,y) => z
           ; etc

would be more clearly expressed as:

     MyRule: x -> y
       where z := <foo>(x, y)
           ; etc.