 A development for formula_1 is a countable collection formula_3 of open coverings of formula_1, such that for any closed subset formula_5 and any point formula_6 in the complement of formula_7, there exists a cover formula_8 such that no element of formula_8 which contains formula_6 intersects formula_7.A space with a development is called developable.A development formula_12 such that formula_13 for all formula_14 is called a nested development.