$e = x \mid \lambda x\!:\!\tau.e \mid e \, e \mid c$
So, what is $\mid$ in this example of simply typed lambda calculus?
The syntax of the simply typed lambda calculus is essentially that of the lambda calculus itself. (Wikipedia, simply typed lambda calculus)
I wasn't able to see $\mid$ in untyped lambda calculus.. How is $\mid$ used in lambda calculus?