Computation is a powerful artistic medium. Artists with experience in programming have demonstrated the unique creative opportunities of using code to make art. Currently, manual artists interested in using procedural techniques must undergo the difficult process of learning to program, and must adopt tools and practices far removed from those to which they are accustomed. We hypothesize that, through the right direct manipulation interface, we can enable accessible and expressive procedural art creation. To explore this, we developed Para, a digital illustration tool that supports the creation of declarative constraints in vector artwork. Para's constraints enable procedural relationships while facilitating live manual control and non-linear editing. Constraints can be combined with duplication behaviors and ordered collections of artwork to produce complex, dynamic compositions. We use the results of two open-ended studies with professional artists and designers to provide guidelines for accessible tools that integrate manual and procedural expression.