The core objective of the Physics Derivation Graph is identifying the mathematical connectivity of the various domains of Physics. The same connectivity should be able to relate "basic" Physics (e.g., F=ma) to advanced Physics (e.g., the theory of the Standard model, string theory).
I know of two ways to document the connectivity:
identify symbol re-use
identify inter-related derivations at the level of expressions
The symbol re-use is less interesting than the re-use of expressions.
but I didn't specify what would qualify as sufficient to show completeness.
What core expressions are representative of all of Physics? Are there a set of expressions that, if shown to be connected by derivations, would be sufficient to demonstrate the span of the concept?
I expect that, for any given subdomain of Physics, there are central expressions. The task list is then
identify every named expression in Physics
identify every variable in that list of named expressions
enter all named expressions and variables into the PDG database
determine which expressions are not connected to anything