_
    _|
  _|
_|

stair(1)  ➞ "  _\n_|"
# 2 spaces, 1 underscore, 1 newline, 1 underscore, 1 vertical line

stair(2)  ➞ "    _\n  _|\n_|"
# 4 spaces, 1 undescore, 1 newline, 2 spaces, 1 underscore,
# 1 vertical line, 1 newline, 1 underscore, 1 vertical line

stair(3) ➞ "      _\n    _|\n  _|\n_|"
# 6 spaces, 1 undescore, 1 newline, 4 spaces, 1 underscore,
# 1 vertical line, 1 newline, 2 spaces, ...

stair(4) ➞ "        _\n      _|\n    _|\n  _|\n_|"
# 8 spaces, 1 undescore, 1 newline, 6 spaces, 1 underscore,
# 1 vertical line,  ...

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