consider fs1 being object such that
A1: fs1 in R1 by XBOOLE_0:def 1;
consider fs2 being object such that
A2: fs2 in R2 by XBOOLE_0:def 1;
reconsider fs1 = fs1, fs2 = fs2 as firing-sequence of N by A1, A2;
fs1 ^ fs2 in R1 before R2 by A1, A2;
hence not R1 before R2 is empty ; :: thesis: verum