Unable to use an arbitrary expression for an index value under Systemtap

Issue

• The following Systemtap script returns a parse error, when an arbitrary expressions is used for an index value:

# cat ./example.stp
#!/usr/bin/stap

probe module("ext4").function("ext4_set_acl") {
        if ($inode->i_ino == $1) {
                for (i = 0; i < $acl->a_count; i++) {
                        if (($acl->a_entries[$i]->e_tag) & 0x2) {
                                        printf ("\tacl: %s\n", $acl->a_entries[$i]$$)
                        }
                }
        }
}
# ./example.stp 157922
parse error: expected number
        saw: identifier '$i' at ./example.stp:5:38
     source:                 if (($acl->a_entries[$i]->e_tag) & 0x2) {
                                                                 ^
1 parse error(s).
Pass 1: parse failed.  Try again with another '--vp 1' option.
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