  Let R be a countable, principal ideal domain which is not a field and A be a
countable R-algebra which is free as an R-module. Then we will construct an
aleph_1-free R-module G of rank aleph_1 with endomorphism algebra End_RG=A .
Clearly the result does not hold for fields. Recall that an R-module is
aleph_1-free if all its countable submodules are free, a condition closely
related to Pontryagin's theorem. This result has many consequences, depending
on the algebra A in use. For instance, if we choose A=R, then clearly G is an
indecomposable `almost free' module. The existence of such modules was unknown
for rings with only finitely many primes like R=Z_{(p)}, the integers localized
at some prime p. The result complements a classical realization theorem of
Corner's showing that any such algebra is an endomorphism algebra of some
torsion-free, reduced R-module G of countable rank. Its proof is based on new
combinatorial-algebraic techniques related with what we call rigid
tree-elements coming from a module generated over a forest of trees.
