Thanks for the additional context / details. You'll probably want to add items to stimulus (<text>, <picture>) or <item> elements at runtime to handle the repetitions in the same prime-exposure condition. Below a stripped-down example where 'a' and 'b' trials represent (here only two) different exposure durations (a=500ms; b=1000ms):
<block myblock>
/ trials = [1-4=noreplace(a_1, b_1); 5-8=noreplace(a_2, b_2)]
</block>
<trial a_1>
/ ontrialend = [text.p_a.item=text.p_1.currentitem]
/ stimulustimes = [1=p_1; 500=blank]
/ validresponse = (57)
</trial>
<trial b_1>
/ ontrialend = [text.p_b.item=text.p_1.currentitem]
/ stimulustimes = [1=p_1; 1000=blank]
/ validresponse = (57)
</trial>
<trial a_2>
/ stimulustimes = [1=p_a; 500=blank]
/ validresponse = (57)
</trial>
<trial b_2>
/ ontrialend = [text.p_b.item=text.p_1.currentitem]
/ stimulustimes = [1=p_b; 1000=blank]
/ validresponse = (57)
</trial>
<text p_1>
/ items = ("A", "B", "C", "D")
/ select = noreplace
</text>
<text p_a>
/ select = noreplace
</text>
<text p_b>
/ select = noreplace
</text>
<shape blank>
/ shape = rectangle
/ color = (white)
/ size = (100%, 100%)
</shape>