defining a constant



Hi,

how do I define a constant in Maxima?  Is the proper way to use
DEFINE_VARIABLE with a mode of ANY_CHECK and a function that always
errors out?

The info manual entry

,----
| Info from file /usr/local/share/info/maxima.info:
|  - special operator: CONSTANT
|      - makes ai a constant as is %PI.
`----

made me believe that I might define some constant by calling CONSTANT
with some arguments, but this does not seem to be the case.  Indeed, I
do not see where $CONSTANT is defined in the Lisp sources, and sure enough

,----
| (%i14) ?describe(constant);
| 
| $CONSTANT is an internal symbol in the MAXIMA package.
| (%o14)                               FALSE
`----

it does not seem to be.

Thanks in advance for any clarifications,

Albert.

P.S.: Version: Maxima 5.9.1.