  Assume a (semi)parametrically efficient estimator is given of the Euclidean
parameter in a (semi)parametric model. A submodel is obtained by constraining
this model in that a continuously differentiable function of the Euclidean
parameter vanishes. We present an explicit method to construct
(semi)parametrically efficient estimators of the Euclidean parameter in such
equality constrained submodels and prove their efficiency. Our construction is
based solely on the original efficient estimator and the constraining function.
  Only the parametric case of this estimation problem and a nonparametric
version of it have been considered in literature.
