fact: Definition zero_triple: (val * (val * (val*val))) := (nullval, (nullval, (nullval,nullval))).
imports: VST.veric.rmaps CertiGraph.lib.List_ext CertiGraph.graph.graph_model CertiGraph.CertiGC.GCGraph CertiGraph.CertiGC.spatial_gcgraph CertiGraph.CertiGC.env_graph_gc CertiGraph.msl_ext.iter_sepcon