Example:

    class Fire{}
    class Water {}
    class Grass {}
    class Ice {}
    
    def eff(Fire f, Grass g) { 'burn' }
    def eff(Fire f, Ice ice) { 'melt' }
    def eff(Water w, Grass g) { 'grow' }
    def eff(Water w, Ice ice) { 'freeze' }
    def eff(a, b) { 'undefined' }
    
    println eff(new Fire(), new Grass())
    println eff(new Fire(), new Ice())
    println eff(new Water(), new Grass())
    println eff(new Water(), new Ice())
    
    println eff(new Ice(), new Water())

Prints:

burn melt grow freeze undefined