  Let $\PP^d$ be the $d$-fold direct product of the set of primes. We prove
that if $A$ is a subset of $\PP^d$ of positive relative upper density then $A$
contains infinitely many "corners", that is sets of the form
$\{x,x+te_1,...,x+te_d\}$ where x is an integer point and e_1,...,e_d are the
standard basis vectors of the d-dimensional Euclidean space. Our argument is
based on proving a removal lemma for weighted uniform hypergraphs, where the
weight system is defined in terms of a pairwise linearly independent family of
linear forms.
