extrap — extrapolate a ts by another ts
tsout=extrap(arg1, argn)
if x is a ts defined over the time span '10a'-'20a', y a ts defined over the time span '19a-28a' and z a ts defined over the time span '1a'-'35a': 1) a = extrap(x,y) 2) b= extrap(x,y,z) 3) c= overaly(x,z) a is a ts defined over the time span '10a'-'28a', equal to x over the time span '10a'-'20a', y*value(x/y,'20a') over the time span '21a'-'28a'. b is a ts defined over the time span '10a'-'35a', equal to x over the time span '10a'-'35a', y*value(x/y,'20a') over the time span '21a'-'28a', z*value(y/z,'25a')* value(x/y,'20a') over the time span '29a'-'35a'. c is a ts defined over the time span '10a'-'35a', equal to x over the time span '10a'-'20a', y*value(x/y,'20a') over the time span '21a'-'35a'.