A library for expressing "spreadsheet-like" computations with absolute and relative references, using fixed-points of n-dimensional comonads. A sheet is an n-dimensionally nested Tape, which is a stream infinite in both left and right directions, with a focus element. For instance, type Sheet1 a = Nested (Flat Tape) a, which is isomorphic to Tape a. Nested Tapes describe multi-dimensional grid-lik