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aTHX my_perl aTHX_ my_perl, ... pTHX_ PerlInterpreter* my_perl, dTHX PerlInterpreter* my_perl = something_that_retrieves_the_current_perl();
void foo(pTHX_ SV * sv) { ... }
foo(aTHX_ some_sv);
void foo(SV * sv) { dTHX; ... }