  Version 2: Corrected Section 3.3: instead of lexicographical normal forms we
now use a normal form due to V. Diekert and A. Muscholl. Consequent changes
made and some misprints corrected.
  Using an analogue of Makanin-Razborov diagrams, we give an effective
description of the solution set of systems of equations over a partially
commutative group (right-angled Artin group) $G$. Equivalently, we give a
parametrisation of $Hom(H, G)$, where $H$ is a finitely generated group.
