rubytapas coerce

  def coerce(other)
    unless other.is_a?(Numeric)
      raise TypeError, "Can't coerce #{other}" 
    end
    
    operator = caller.first.scan(/`(.*)'/).first.first.to_sym
    
    return [Feet.new(other), self] if operator == :*

    raise TypeError "Can't coerce #{other} for operator '#{operator}'"
  end