Class placeholder:
     Def __init__(self, name):
      Self.name=name

    Class suspended:
      Def __init__(self, f, name):
        Self.f=f
        Self.name=name

    class concreteValue(rhs):
      Def __init__(self, x):
         Self.x=x

    Def bind(x, f):
     If is instance(x, concreteValue):
        Return f(x)
     Elif is instance(x, placeholder):
       Return suspended(f, x.name)
     Elif is instance( x, suspended):
       Return suspended(lambda val: f(x.f(val)), x.name