  The Wigner-Araki-Yanase (WAY) theorem states a remarkable limitation to
quantum mechanical measurements in the presence of additive conserved
quantities. Discovered by Wigner in 1952, this limitation is known to induce
constraints on the control of individual quantum systems in the context of
information processing. It is therefore important to understand the precise
conditions and scope of the WAY theorem. Here we elucidate its crucial
assumptions, briefly review some generalizations, and show how a particular
extension can already be obtained by a simple modification of the original
proofs. We also describe the evolution of the WAY theorem from a strict no-go
verdict for certain, highly idealized, precise measurements into a quantitative
constraint on the accuracy and approximate repeatability of imprecise
measurements.
