空格的 ”陷阱“

今天無意中碰到一個空格帶來的“陷阱”：

def a
  10
end

b = a+1
=> 11

b = a + 1
=> 11

b = a+ 1
=> 11

b = a +1
ArgumentError: wrong number of arguments (1 for 0)
        from (irb):8:in `a'
        from (irb):8