  This manuscript studies actions of communication between epistemic logic
agents. It starts by looking into actions through which all/some agents share
all their information, defining the model operation that transforms the model,
discussing its properties, introducing a modality for describing it and
providing an axiom system for the latter. The main part of the manuscript
focuses on an action through which some agents share part of their information:
they share all that they know about a topic defined by a given formula. Once
again, the manuscript defines the model operation that transforms the model,
discusses its properties, introduces a modality for describing it and provides
an axiom system for the latter.
