-# When compiling with GCC on some OSs (Solaris, AIX), _Complex_I doesn't work;
-# in the reported cases so far, 1.0fi works well instead.
+# When compiling with GCC on some OSs (Solaris, AIX), _Complex_I doesn't
+# work; in the reported cases so far, 1.0fi works well instead. According
+# to the C99 spec, the complex.h header must provide a working definition
+# of _Complex_I, so we always try _Complex_I first. The 1.0fi fallback
+# is a workaround for the failure of some systems to conform to C99.