Ordered data and numeric conditions

world open.
step1:Step[position -> 1; next -> step2].
step2:Step[position -> 2; next -> step3].
step3:Step[position -> 3].
?Step:EarlyStep <- ?Step:Step[position -> ?Position]
                  AND ?Position < 3.
?- ?Step:EarlyStep.
?- step1[next -> ?Next].
expect true: step2:EarlyStep.

The first query returns step1 and step2; the second returns step2. The numeric variable is bound by a frame before it is compared.

A sequence can be modeled with named nodes, position values and next edges. This finite graph is inspectable and can carry provenance. It does not implement Prolog list terms, head/tail unification or arbitrary term construction.

The next edge describes an order. Merely asserting it does not execute a step, consume a token or prevent two steps from running. An external executor must respect the permissions and transitions derived by the rules.

Use rules to compare values that are already available. FrameX 0.4.4 also supports arithmetic bindings such as ?Next := ?Position + 1 once the input is bound. Parsing and external access belong in explicit adapters with recorded inputs and outputs. Aggregates are outside this core teaching profile.

Try it

Add a fourth step. Which facts describe ordering, and which rule classifies early steps? Does either perform an action?