def bind(param, target)
      raise ScopeError unless scope.include? target
      @_params.fetch(param).bind target
    end
def bind(param, target)
      raise ScopeError unless scope.include? target
      @_params.fetch(param).bind target
    end
Binds a parameter to a target value.

 @see    Parameter#bind
 @raise  [KeyError] if the parameter is not found.
 @raise  [ScopeError] if the parameter is out of scope.

 @param  param [Symbol] the parameter to bind.
 @param  target [#value] the value object to bind to.
 @return [Parameter] the parameter that was bound.