v2.22.0

INDIRECT reads R1C1 references, and ADDRESS joins the engine to write
them.

New

• INDIRECT honours its second argument. With FALSE the text is read as
  R1C1: absolute parts such as R2C3, relative ones such as R[-1]C[2], and a
  bare R or C for the formula's own row or column. Relative parts are
  measured from the cell holding the formula, a sheet prefix is kept, and a
  two-cell range works inside an aggregate.
• ADDRESS, which was missing entirely. It writes an A1 or R1C1 reference
  for a row and column, with all four anchoring modes and an optional sheet
  name, quoted when the name needs it. INDIRECT(ADDRESS(...)) round trips in
  both styles.

Fixed

• INDIRECT used to ignore its second argument, so INDIRECT("R2C3", FALSE)
  was read as A1 text and failed with #REF!.

Notes

A reference that would fall off the grid, such as R[-1]C in row 1, is
#REF!, and A1 text passed with FALSE is rejected rather than quietly read
the other way. Whole-row and whole-column R1C1 forms (R2, C3) are not
supported yet.

That takes the engine to 322 function names.