The ceil command is a part of the TclX extension. TclX created command interfaces to a number of Tcl mathematical functions. So just read the expr documentation for ceil, and you will know what kind of arguments to pass to it.
RS: Of course you can also write, if you wish
proc ceil x {expr {ceil($x)}}so you don't have to require TclX...
LV Certainly. However, the point was to document what the TclX ceil command does...