Support program modules

We want to have some kind of module, which is essentially a self-contained program with its own EDB and IDB predicates, separate from the rest. Invoking the module provides a mapping from predicates on the outside to EDB predicates for the module, and potentially from some IDB predicates of the module back to some predicates of the outside. The details of syntax and semantics are yet to be determined.

The issue proposes self-contained program modules with separate EDB and IDB predicates and mappings to and from the surrounding program. No files, tests, syntax, or semantics are specified; first clarify the module design and identify the relevant language entry points before implementation, with completion defined by agreed syntax, semantics, and coverage for module invocation and predicate mappings.

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