maximilianriemensberger cb4bf26fc7 Fix compilation error with GCC 5 ...

GCC 5 complains that `operator-=` and `operator+=` are not constexpr.  The `-` and `+` operator appear seem to be fine.

2019-09-03 22:59:04 -04:00

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Fix compilation error with GCC 5

2019-09-03 22:59:04 -04:00