macro 

rse x1 x2 y

mcolumn x1 x2 y z
mconstant nrand

let nrand=count(x1)
rand nrand z;
normal 0 .6.
let y=18.5+2.376*x1+1.2*x2-.0234375*x1*x1-.02*x2*x2+.0125*x1*x2+z

endmacro