It seems that SRE initially kind of supported atomic groups. At least it was supported in the compiler and in the interpreter, but not in the parser. And opcode CALL was added for this. In 43b3b49 the support of opcode CALL was removed from the interpreter, but it was left in the compiler.

A new opcode ATOMIC_GROUP was added to support atomic groups, so now we can remove opcode CALL.

Alternatively we can rename ATOMIC_GROUP to CALL.