The question of Greece's public debt stock "is not taboo" but its primary budget surplus is an essential question that must be dealt with first in talks with its international lenders, French Finance Minister Michel Sapin said on Thursday. "Let's first look at where we stand with the primary surplus, what measures are needed for the surplus to be big enough, stable, coherent and then, afterwards, starting from there, we'll consider the debt," he told a news conference.