16. A computer-aided design system comprising: a processor operatively coupled to a data storage system, the data storage system storing a three-dimensional model; and a data storage memory operatively coupled to the processor and comprising instructions to configure the processor to: construct a plurality of model features defining the three-dimensional model; recognize that the plurality of model features produce geometry giving rise to an implicit feature, the implicit feature being defined implicitly in terms of existing external features; calculate variations of the implicit feature; present geometry produced by the variations of the implicit feature to allow selection of one of the variations; create an explicit feature that produces geometry giving rise to the selected one of the variations of the implicit feature; and replace in the three-dimensional model geometry giving rise to the implicit feature with geometry produced by the explicit feature.
16. A computer-aided design system comprising: a processor operatively coupled to a data storage system, the data storage system storing a three-dimensional model; and a data storage memory operatively coupled to the processor and comprising instructions to configure the processor to: construct a plurality of model features defining the three-dimensional model; recognize that the plurality of model features produce geometry giving rise to an implicit feature, the implicit feature being defined implicitly in terms of existing external features; calculate variations of the implicit feature; present geometry produced by the variations of the implicit feature to allow selection of one of the variations; create an explicit feature that produces geometry giving rise to the selected one of the variations of the implicit feature; and replace in the three-dimensional model geometry giving rise to the implicit feature with geometry produced by the explicit feature. 20. The computer-aided design system of claim 16 , further comprising instructions to construct the three-dimensional model by creating the geometry giving rise to the implicit feature prior to creating the explicit feature, wherein creating the explicit feature comprises instructions to produce geometry to replace the geometry giving rise to the implicit feature.