 Complete the following phrase:  While started to work in 1973 before the Spanish withdrawal from Western Sahara, its official foundation was in 1975. Initially an important part of POLISARIO, as much of the Sahrawis who joined the movement in the first years were Fos Bucraa workers, its importance has diminished as a result of the mass exodus of Sahrawis, mostly in the 1975-76 period, at the beginning of the 