GHC CVS compiling problem

helo
i'm experiencing a problem when trying to compile the CVS tree of ghc.
i've COed from the cvs the modules "fpconfig", "ghc" and "libraries".
after having created a "fptools-working" directory with the "lndir" tool
and set up mk/build.mk (taken from mk/build.mk.sample) 
, i run "make -f Makefile.config" without problems,
i've run "autoconf && (cd ghc; autoconf)" successfully, 
but "./configure --prefix=$TOP/build" fail.

the useful part of the output is:
-------
checking for alex... no
checking for version of alex... grep: ./alex/mk/version.mk: No such file
or directory
configure: error: Alex version 2.0 or later is required to compile GHC.
-------

it seems to me that a tool called "alex" is missing, but i don't know
what this tool can be. it isn't written anywhere in the build help, in
the FAQ or elsewhere. and i failed to find a software with this name on
google and freshmeat.

so, can anyone help me ?

P.S i'm running a debian sid with ghc6, gcc-2.95, happy, haddock, flex
and the required tools. 

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