\@for
流れ
\def\somecs#1,#2,#3\@nil{
#1
\edef\@tempa{#2}
\ifx\@tempa\empty
\else
\somecs#2,#3,\@nil
\fi
}
のようなものを作って
\somecs A,B,C,D,,\@nil
等の処理を考える.
\somecs A,B,C,D,,\@nil
#1 = "A", #2 = "B", #3 = "C,D,"
\somecs B,C,D,,,\@nil
#1 = "B", #2 = "C", #3 = "D,,,"
\somecs C,D,,,,,,\@nil
#1 = "C", #2 = "D", #3 = ",,,,,"
\somecs D,,,,,,,,,\@nil
#1 = "D", #2 = "", #3 = ",,,,,,,"
\somecs A,B,C,,\@nil
#1 = "A", #2 = "B", #3 = "C,,"
\somecs B,C,,,,,\@nil
#1 = "B", #2 = "C", #3 = ",,,,"
\somecs C,,,,,,,,\@nil
#1 = "C", #2 = "", #3 = ",,,,,,"
\somecs A,B,,\@nil
#1 = "A", #2 = "B", #3 = ","
\somecs B,,,,,\@nil
#1 = "B", #2 = "", #3 = ",,,"
\somecs A,,\@nil
#1 = "A", #2 = "", #3 = ""