I'm pretty sure that this was supported at some time in the past - possibly in the pre 2.2 era, don't know

This is for instance a trigger expression where I'm sure that once worked:

{$FS_SIZE_THRESHOLD}=>25G

{example.com:vfs.fs.size[/,total].last(0)}>{$FS_SIZE_THRESHOLD}-1&{example.com:vfs.fs.size[/,free].last(0)}<{$FS_SIZE_FREE}

Now it's unsupported due to format error:

Format error or unsupported operator. Exp: [25GN1]