Stacking decorators - order of application

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Decorators stack bottom-up: the one closest to the function is applied first. @add_x over @add_y means: first wrap base with add_y's wrapper, then wrap that result with add_x's wrapper. So calling base() actually calls add_x_wrapper(), which calls add_y_wrapper(), which calls the original base() returning 'Z'. add_y appends 'Y' to get 'ZY', then add_x appends 'X' to get 'ZYX'. A useful mental model: read the decorators from bottom to top when reasoning about execution order.