Considering, that I have a set of atoms:

Fe1, Ti2, Fe3, Fe4

I want to set following occupancy constraints in Rietveld refinement in GSAS/EXPGUI.

First two are relatated to chemical composition:

Fe1+Fe3=1

Ti2+Ti4=1

Other two are related to the same position in the structure:

Fe1+Ti3=1

Fe3+Fe4=1

Unfortunately when I try to input this constrains, I recieve an error which says that atomic parameter can appear in only one constraint.

Do you know any solution or trick how to solve this?