sentences:  Moreover, formula_87 is an isometric immersion and Lemmas 5,6, and 8 show the existence of a parametrization formula_77 of the whole formula_6, such that the coordinate curves of formula_66 are the asymptotic curves of formula_6.This result was provided by Lemma 4.Therefore, formula_6 can be covered by a union of "coordinate" quadrilaterals formula_93 with formula_94.