What does =+ (equals-plus) mean in C?

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In ancient versions of C, =+ was equivalent to +=. Remnants of it have been found alongside the earliest dinosaur bones.

For example, B introduced generalized assignment operators, using x+=y to add y to x. The notation came from Algol 68 via McIlroy, who incorporated it in his version of TMG. (In B and early C, the operator was spelled =+ instead of +=; this mistake, repaired in 1976, was induced by a seductively easy way of handling the first form in B’s lexical analyzer.)

[The Development of the C Language, Dennis Ritchie. Copyright ACM, 1993. Internal citations omitted.]

Since the mid-1970’s, it has no special meaning — it’s just a = followed by a +.

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