The development of a family of applications in a domain can be greatly eased if patterns in the domain are systematically reused. Systematic use of such a pattern can be achieved by tools that support pattern-based development. In this paper, we describe a prototype tool called RBMLPattern Instantiator (RBML-PI) that generates UML class diagrams and sequence diagrams from a pattern specification described in the Role-Based Metamodeling Language (RBML), a pattern specification language, that defines a domain-specific sub-language of the UML. We give an overview of the RBML using the Observer design pattern, and demonstrate the tool using an RBML specification for the CheckIn-CheckOut (CICO) domain pattern that characterizes services to check in and check out items. We use the CICO pattern specification to generate UML models of a library system and a vehicle rental system using RBML-PI.