Expression selectors and reduction bindings

Within an expression, X[loc, age:ap] keeps the current loc selection while binding the age position to the reduction variable ap. For example:

total[loc] = apply_along(X[loc, age:ap], age=ap, kernel=sum)

loc comes from the alias's left-hand side; ap visits the declared age coordinates. Nested reductions can bind additional axes without dropping the unreduced group selector. The resulting total stays local to each group, so a group block evaluator can retain its independence.

Use axis:variable inside expression brackets and axis=variable in the reducer's keyword arguments. Historical examples that put axis=variable inside a state subscript are not valid expression syntax in the typed-IR compiler. Rewrite them using colon selectors and an expression-first reducer call as above. Invalid alias expressions fail with the alias's context instead of becoming partially expanded state names.

Initial-state parameter names and expanded transition labels retain a separate compatibility string-substitution path. When that path binds a wildcard in a mixed selector, it preserves all explicitly bound entries; it must not collapse the whole selector into a cell name containing only the wildcard axis. This compatibility behavior does not make equals selectors valid expression syntax.