Clarify use of undefined Exprs in BoundaryConditions
Clarify use of undefined Exprs in BoundaryConditions


Former-commit-id: a1cdd332d20253a1c5289bb0b196c35ba9ac95fc
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide