Defining operators including vertical bars(|) in SWI-prolog

Question

I am trying to encode basic logical inferences in Prolog, and I want to define some custom operators to streamline the notation. It would be handy if I can type |- for ⊢. So I tried

:- op(1150, xfy, [ '|-' ]).
gamma |- a.
Gamma |- or(A,_) :- Gamma |- A.
Gamma |- or(_,A) :- Gamma |- A.

but when I try the query gamma |- or(a,X), I get the error message

ERROR: '<meta-call>'/1: Undefined procedure: gamma/0

instead of the true I expect.

The problem seems to be that the defined operator includes a vertical bar character. If I modify the knowledge base to

:- op(1150, xfy, [ imp ]).
gamma imp a.
Gamma imp or(A,_) :- Gamma imp A.
Gamma imp or(_,A) :- Gamma imp A.

Then Prolog has no problems answering the query gamma imp or(a,X). Is the vertical bar a reserved character that I'm not allowed to use in definitions? Or is there some way around this?