  Difficult it is to formulate achievable sensitivity bounds for quantum
multiparameter estimation. Consider a special case, one parameter from many:
many parameters of a process are unknown; estimate a specific linear
combination of these parameters without having the ability to control any of
the parameters. Superficially similar to single-parameter estimation, the
problem retains genuinely multiparameter aspects. Geometric reasoning
demonstrates the conditions, necessary and sufficient, for saturating the
fundamental and attainable quantum-process bound in this context.
