Mono fails to build in Gentoo

Problem: On Gentoo systems mono fails to build when performing an update (emerge -u world) (Mar 2007)

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Got a SIGABRT while executing native code. This usually indicates
a fatal error in the mono runtime or one of the native libraries
used by your application.
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[SOLVED]

Mono fails to emerge if an older version is installed. Uninstall (unmerge) the old mono version and emerge the new one.

~# emerge -p -C mono

~# emerge -p mono

Remove the "-p" praefix to make it work!