  Real world applications of planning, like in industry and robotics, require
modelling rich and diverse scenarios. Their resolution usually requires
coordinated and concurrent action executions. In several cases, such planning
problems are naturally decomposed in a hierarchical way and expressed by a
Hierarchical Task Network (HTN) formalism. The PDDL language used to specify
planning domains has evolved to cover the different planning paradigms.
However, formulating real and complex scenarios where numerical and temporal
constraints concur in defining a solution is still a challenge. Our proposition
aims at filling the gap between existing planning languages and operational
needs. To do so, we propose to extend HDDL taking inspiration from PDDL 2.1 and
ANML to express temporal and numerical expressions. This paper opens
discussions on the semantics and the syntax needed to extend HDDL, and
illustrate these needs with the modelling of an Earth Observing Satellite
planning problem.
