checks to see which event occurred first A or B

event pseudocode

a_occured = false
b_occured = false

function event(source){
    if(source==A){
        if(b_occured==false){
            a_occurred=true
        }
    }else{
        if(a_occured==false){
            b_occurred=true
        }
    }
}

if (a_occured==true){
    print "A occurred first"
}
if (b_occured==true){
    print "B occurred first"
}