private Maybe<Result> VisitOp(QueryParser.Bool_exprContext context)
{
var left = new BasicExprVisitor(_zmi, _env).Visit(context.basic_expr()[0]);
var right = new BasicExprVisitor(_zmi, _env).Visit(context.basic_expr()[1]);
return left.Zip(right)
.Bind(tuple => new RelOpVisitor((tuple.Item1, tuple.Item2)).Visit(context.rel_op()));
}
