BB-7: Query global execution mode

[mcmillan03]

To query the mode, for example (from Suitesparse:GraphBLAS):

GrB_mode mode ;
GxB_get (GxB_MODE, &mode) ;

[tgmattso]

Yes. Full introspection is needed. For every object defined in GraphBLAS and every mode, you can query its value (a GrB_get())

[manoj63]

Seems a reasonable philosophy. Anything that can be set by an application should be query-able also, at least for debugging.