x1 is the conceptualized/ideal/abstract shape of a biconvex lens formed by/manifested from the intersection x2 (parameters) of two-dimensional circular disks immersed (embedded) in geometry/defined by metric x3; x1 is the convex-only region bounded by intersecting circular arcs given by x2
x1 (shape/form) is (idealized as) a quadrilateral/two-dimensional closed shape bounded by exactly four distinct non-colinear line-segments (or circular arcs), with vertices x2 (ordered cyclical list) and with additional properties x3.