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...

[DKF]: Also available in Tcl 8.5 as '''[tcl::mathfunc]::ceil'''.

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