[Gretl-users] why the error

dear everyone, when i write in gretl the scripts " open greene22_2 discrete Z5 list X=dummify(Z5) --drop-first " then I run the program and get the results, " gretl version 1.9.92 Current session: 2015-03-14 16:31 ? open greene22_2 Read datafile C:\Program Files\gretl\data\greene\greene22_2.gdt periodicity: 1, maxobs: 601 observations 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 The symbol 'drop' is undefined Error executing script: halting " why did i encounter the error? best, shi,qi