[Gretl-users] why the error?

dear everyone, when i write in gretl the scripts "open greene22_2discrete Z5list X=dummify(Z5) --drop-first" then I run the program and get the results, "gretl version 1.9.92Current session: 2015-03-14 16:31 ? open greene22_2 Read datafile C:\Program Files\gretl\data\greene\greene22_2.gdtperiodicity: 1, maxobs: 601observations range: 1 to 601 Listing 10 variables: 0) const 1) Y 2) Z1 3) Z2 4) Z3 5) Z4 6) Z5 7) Z6 8) Z7 9) Z8 ? discrete Z5? list X=dummify(Z5) --drop-first> list X=dummify(Z5) --drop-The symbol 'drop' is undefined Error executing script: halting> list X=dummify(Z5) --drop-first" why did i encounter the error? best, shi,qi