added on in prepositions

ps-grammar/gprs/gprs.jsgf

@@ -47,7 +47,7 @@ public <task> = [tobi] [(would | could) you] [please] <command> [please] [tobi];
 <utensil_class> = trash_bin;
 <exit_class> = (exit door) | (entry door);
 
-<preposition> = into | to | from;
+<preposition> = into | to | from | on;
 <further_preposition> = (on [top of]) | in | (next to) | at | near;
 
 <article> = the | an | a | some | this;