[FOM] AEC

[pax0 at seznam.cz]

Hi again those familiar with Shelahs two-volume book on AEC.
 in 1st volume
on page 261
 (b) monotonicity
 why he says

 so it is legitimate to just say "tp(a,M_1,M_3) does not fork over M_0"
 Can you give a more detailed explanation why we can do so?
 Thank you, Jan