  Elliptic multiple polylogarithms occur in Feynman integrals and in particular
in scattering amplitudes. They can be characterized by their symbol, a tensor
product in the so-called symbol letters. In contrast to the non-elliptic case,
the elliptic letters themselves satisfy highly non-trivial identities, which we
discuss in this paper. Moreover, we introduce the symbol prime, an analog of
the symbol for elliptic symbol letters, which makes these identities manifest.
We demonstrate its use in two concrete examples at two-loop order: the
unequal-mass sunrise integral in two dimensions and the ten-point double-box
integral in four dimensions. Finally, we also report the result of the
polylogarithmic nine-point double-box integral, which arises as the soft limit
of the ten-point integral.
