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The PRE_BUILD file is executed by cast before the BUILD file.
Anything necessary to prepare the spell to be built should be done here, like patching or unpacking additional sources. Usually this is done by adding commands after default_pre_build, which unpacks the first source automatically:
default_pre_build && cd "$SOURCE_DIRECTORY" && unpack_file 2 && patch -p0 < "$SPELL_DIRECTORY/gcc4.patch"
Another use for this file is for spells using badly packaged sources that don't unpack into a directory:
mk_source_dir "$SOURCE_DIRECTORY" && cd "$SOURCE_DIRECTORY" && unpack_file