st: simultaneous equation with qualitative variables

["lfq" <stata_lfq@sina.com>]

how can i use stata to estimate  simultaneous equations with qualitative variables as follows:
y1=a1+b1y2+c1x1+e1
y2=a2+b2y1+c2x2+e2
y1*=1 if y1>0
y1*=0 if y1<=0
y2*=1 if y2>0
y2*=0 if y2<=0

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