  In the classic Coleman--Mandula no-go theorem which prohibits the unification
of internal and spacetime symmetries, the assumption of the existence of a
positive definite invariant scalar product on the Lie algebra of the internal
group is essential. If one instead allows the scalar product to be positive
semi-definite, this opens new possibilities for unification of gauge and
spacetime symmetries. It follows from theorems on the structure of Lie
algebras, that in the case of unified symmetries, the degenerate directions of
the positive semi-definite invariant scalar product have to correspond to local
symmetries with nilpotent generators. In this paper we construct a workable
minimal toy model making use of this mechanism: it admits unified local
symmetries having a compact -- U(1) -- component, a Lorentz -- SL(2,C) --
component, and a nilpotent component gluing these together. The construction is
such that the full unified symmetry group acts locally and faithfully on the
matter field sector, whereas the gauge fields which would correspond to the
nilpotent generators can be transformed out from the theory, leaving gauge
fields only with compact charges. It is shown that already the ordinary Dirac
equation admits an extremely simple prototype example for the above gauge field
elimination mechanism: it has a local symmetry with corresponding eliminable
gauge field, related to the dilatation group. The outlined symmetry unification
mechanism can be used to by-pass the Coleman--Mandula and related no-go
theorems in a way that is fundamentally different from supersymmetry. In
particular, the mechanism avoids invocation of super-coordinates or extra
dimensions for the underlying spacetime manifold.
