= Solution
The qualitative prediction of <galactic chemical evolution> is useful: later <stellar populations> tend to inherit more enriched gas, and old <stellar populations> can preserve low <stellar metallicity>. The <closed-box metallicity distribution>, however, predicts many more metal-poor long-lived dwarfs in the solar neighbourhood than are observed. This is the <G-dwarf problem>, also apparent with other long-lived tracers. A single well-mixed box cannot describe the different age and abundance distributions of the <galactic disk>, <galactic bulge>, and <stellar halo> of the <Milky Way>. Nor need its exponentially declining <star formation rate> match the history of ongoing disk <star formation>.
Prolonged relatively metal-poor <galactic gas inflow> supplies fuel while diluting the enrichment, allowing many <stars> to form after the earliest enrichment stage; pre-enrichment of the initial reservoir also removes a pristine tail. <Galactic outflows> lower the <effective yield>, but a constant well-mixed outflow alone still gives an exponential <metallicity distribution function>, and is not automatically a cure for the <G-dwarf problem>. Spatially varying <star formation>, radial transport and migration, and selection of local survivors complicate a comparison with one global box. A varying <initial mass function> or <stellar yield> changes the enrichment law. Finally, finite stellar lifetimes and <Type Ia supernovae> make iron enrichment delayed, so an observed iron-abundance distribution is not interchangeable with the instantaneous total-metal distribution. \b[Gas exchange, spatially distinct populations, and delayed enrichment are essential qualifications to the closed-box prediction.]
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