1. An interface system for providing formally constrained outputs to a computing system in accordance with natural language inputs in a predefined natural language subset, which has a predefined correspondence to said formally constrained outputs, received from an unskilled user, comprising: output means, for indicating to said user a set of permissible items; designating means, for designating, under control of said user, a particular item from among those indicated by said output means; parsing means coupled to said output means and to said designating means, for cumulatively parsing natural language inputs in a form of sequences of items successively designated by said user through said designating means, and for repeatedly generating and indicating on said output means all permissible items which could immediately follow the currently received sequence of items in accordance with said predefined natural language subset; and means coupled to said parsing means for translating the completed input into an executable machine command, in accordance with said predefined correspondence.