对于以下的程序段，关系表达式pice[t1]>pice[t2]被计算的次数是（ ）。
for t1:=1 to 6 do
for t2:=t1+1 to 7 do
if pice[t1]>pice[t2] then
begin w:=pice[t1];pice[t1]:=pice[t2];pice[t2]:=w end;