[ENH] improve subject mask clarity.
[ENH] improve subject mask clarity.

brentp/gemini,xuzetan/gemini,brentp/gemini,bgruening/gemini,udp3f/gemini,arq5x/gemini,bpow/gemini,udp3f/gemini,bw2/gemini,bw2/gemini,bpow/gemini,udp3f/gemini,bpow/gemini,heuermh/gemini,udp3f/gemini,bpow/gemini,xuzetan/gemini,arq5x/gemini,bw2/gemini,arq5x/gemini,heuermh/gemini,bgruening/gemini,arq5x/gemini,bw2/gemini,brentp/gemini,xuzetan/gemini,xuzetan/gemini,bgruening/gemini,brentp/gemini,bgruening/gemini,heuermh/gemini,heuermh/gemini