Neutral oxygen, or the pairing of holes via O^-2, is identified as the essential ingredient of superconductivity in YBa₂Cu₃0^. "Fabrication" of superconducting YBa₂Cu₃O_x from semiconducting PrBa₂Cu₃0₇ in a thought experiment or Gedankenexperiment leads to (i) prediction of the range of superconducting compositions 6.5<x<7, (ii) an understanding of the two-stepped character of the critical temperature T_c(x), (iii) a natural explanation of photoinduced superconductivity, (iv) elucidation of the major doping studies, (v) a derivation of Tokura's empirical rule relating the critical oxygen composition x_c of the alloy У(Ьа₂₇Ва₂__2>,)СизО_х to the critical La content y_c:x_c=6.5 + 2y_c, and (vi) identification of the charge-reservoir layers (the layers containing the Cu-O chains) as the root of superconductivity.