finalize the algebra

(add `inject_interface` to extend stoichiometries "to the right")

src/builders/interface.jl

@@ -100,3 +100,35 @@ Overload of [`interface_constraints`](@ref) for general key-value containers.
 interface_constraints(kv; kwargs...) = interface_constraints(kv...; kwargs...)
 
 export interface_constraints
+
+"""
+$(TYPEDSIGNATURES)
+
+Glue an interface made of additional variables into a constraint system that
+lacks one. Returns a tuple of a "module" and its "interface", usable in
+[`interface_constraint`](@ref).
+
+The values in `interface` are injected into `constraints` that have the same
+"path" in the constraint tree. Attempts to inject into missing `constraints`
+and mismatches between leaf constraints and subtrees are ignored. The output
+consists of the extended `constraints` and `interface` with renumbered
+variables (in particular, numbering of original variables in `constraints` is
+retained). Constraints in both trees retain their bounds.
+
+In metabolic modeling terms, this adds interfaces that contribute to any
+constrained quantity in a model. For example, one may conveniently create new
+exchange reactions to contribute to an existing metabolite balance, which can
+in turn be used as an interface in a more complex model.
+"""
+function inject_interface(constraints::C.ConstraintTree, interface::C.ConstraintTree)
+    go(cs::C.ConstraintTree, iface::C.ConstraintTree) =
+        C.ConstraintTree(k => haskey(iface, k) ? go(v, iface[k]) : v for (k, v) in cs)
+    go(cs::C.Constraint, iface::C.Constraint) =
+        C.Constraint(value = cs.value + iface.value, bound = cs.bound)
+    go(_, _) = cs # any non-matching interface is simply ignored
+
+    interface_ = C.incr_var_idxs(interface, C.var_count(constraints))
+    return (go(constraints, interface_), interface_)
+end
+
+export inject_interface