More research indicates that this may indeed be an issue related to old standards.
See https://en.cppreference.com/w/cpp/language/angle_bracket_hack <https://en.cppreference.com/w/cpp/language/angle_bracket_hack> It indicates that there were issues with some compilers when using standards prior to C++11. Two consecutive “greater than” (>>) signs would be interpreted as the bitwise right shift operator when a templated type (such as complex<double> was used as the template parameter (such as vector<complex<double>>). Hence the error and “suggestion” to put a space between the two “>” signs. This construct is used in multiple places s done in multiple places in Quad_MX.hh). That leads to three possible solutions: Modify Quad_MX.hh to insert a space between the two “>” signs as the compiler is suggesting. That should allow the Mac compilers to compile without the error. I modified the file as a test and it seems to fix the problem. Require Mac users to set the compiler standard (via configure options) so that gnu++11 (or later) is used. This shouldn’t be necessary for gcc users (such as on Linux) as it appears that the default standard setting is gnu++14 dialect if I read the man page correctly. Modify configure to include -std=gnu++11 in CXXFLAGS in the makefiles (works for both Apple and gcc compiler suites) - paul
