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(:=) and (:) are normal binding

(a+a $$ a: 1) == 2
(b $$ b: 3*a // a: 1+1) == 6

x $$ d // e <=> (x $$ d) $$ e

(<-) expands sequences and pattern matches

([x] $$ x<-0,1,2) == [0],[1],[2]
([x] $$ odd x<-0,1,2) == [1]

todo: better to explicitly zip or to use (x<-... :: y<-...) simultaneous def?

(::=) defines algebraic types

(Seq(Lit "a",Rep(Alt(Lit "b",Lit "c"))) $$
    rx ::= Eps | Lit l | Seq(s,t) | Alt(p,q) | Rep(s))

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