code: private WyalFile.Stmt flatten(AssumptionSet assumptions) {
  		WyalFile.Stmt result = flattenUpto(assumptions, null);
  		if (result == null) {
  			return new Expr.Constant(new Value.Bool(true));
  		} else {
  			return result;
  		}
  	}
docs: Flatten a given assumption set into a single logical condition. The key
  challenge here is to try and do this as efficiency as possible.
  
  @param assumptions
  @return