Oh, right you are. It's clearly:

#define SvMAGICAL(sv) (SvFLAGS(sv) & (SVs_GMG|SVs_SMG|SVs_RMG))
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Now, if only I knew what those flags were. Oh, I see. What a wonderful file, thanks.

#define SVs_GMG     0x00002000  /* has magical get method */
#define SVs_SMG     0x00004000  /* has magical set method */
#define SVs_RMG     0x00008000  /* has random magical methods */
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I'm still not clear on what they are, but I have some idea where to start.