Frame

In leading axis theory, the frame is an abstract array-like structure which organizes the k-cells of an array. For k-cells of an array with rank r, the frame consists of the first  axes of the array. The frame concept is most used in working with the Rank operator, or function rank: it is shared between the arguments and result of a function with rank. A concrete array which shares the frame's structure can be produced by applying Enclose or Box with rank k to an array. When representing arrays as nested lists as in K, this array already exists: simply consider the first  levels of nesting to be the array, whose contents are cells.

The frame is complementary to the cell shape for cells of a particular rank: while the frame occupies leading axes (and its shape is a prefix of the array's shape), the cell shape is derived from the trailing axes, so that every axis belongs to either the frame or the cells.