As has been previously discussed, we would like to support x - y between Vector and Matrix objects that behaves like eWiseAdd, but with a default of 0 for the left value if it is not present. This is what GxB_Vector_eWiseUnion and GxB_Matrix_eWiseUnion methods do.

This is currently SuiteSparse:GraphBLAS-specific behavior, but it would be possible to write a recipe that also does this if we ever need to support a non-SuiteSparse implementation.