Showing how to solve the EP in Melange. A (possible) solution to the usual example:

// Base expression language
language BaseExpr {
    syntax "BaseExpr.ecore" // abstract Expr from which Lit/Add inherit
    with EvaluateLit // weaving eval() : int on Lit
    with EvaluateAdd // weaving eval() : int on Add

// New variant
language ExprWithSub inherits BaseExpr {
    syntax "Sub.ecore" // will be merged with BaseExpr.ecore to add Sub

// New operation
language ExprWithPrettyPrint inherits BaseExpr {
    with PrintLit // weaving print() : String on Lit
    with PrintAdd // weaving print() : String on Add

// All variants, all operations
language AllExpr inherits ExprWithSub, ExprWithPrettyPrint {
    // Les deux inherits nous donnent Expr/Lit/Add/Sub
    // avec print() et eval() sur Lit et Add, mais rien sur Sub
    // donc:
    with EvalSub
    with PrintSub

TODO: See how Melange could also handle the Expression Families Problem.