1. A system for sequentially entering data values of a plurality of parameters into an apparatus in response to a series of grammatically correct command text sequences which are displayed by said system; said system comprising: a memory for storing a plurality of partial text sequences each of which relates to one of said parameters, the relevant parameter of a particular sequence being signified by an identifier in said sequence; data processing means for forming a first grammatically correct command text sequence from a first partial text sequence relating to a first of said parameters, said first partial sequence including a first identifier signifying said first parameter; selection means for successively selecting and entering successive data values into successive command sequences in place of the identifiers therein; said data processing means further forming a succession of grammatically correct command sequences respectively relating to successive ones of said parameters following said first parameter, each succeeding command sequence including: i) a partial text sequence relating to the parameter to which said succeeding command sequence relates, said partial sequence including an identifier signifying said parameter; ii) the immediately preceding command sequence, as modified by substitution therein of a data value in place of an identifier, when inclusion of the modified preceding command sequence is consistent with a grammatically correct form of said succeeding command sequence; and iii) when the modified preceding command sequence does not qualify for inclusion under (ii), an alternative form thereof modified so that it does qualify for inclusion under (ii); and a display for sequentially displaying each successive command sequence upon formation thereof.