1. A computer-implemented method for controlling a computer system by providing execution properties in an activity's execution context in a continuation based runtime, the computer-implemented method comprising: an act of instantiating a parent activity configured for execution in a continuation based runtime, the parent activity comprising one or more child activities configured to perform pulses of work, the parent activity being further configured to add execution properties to an execution context, the execution context comprising those execution properties that are available to the parent and child activities during execution; an act of adding one or more execution properties to the parent activity's execution context to generate a modified execution context which includes execution properties that extend the functionality of the parent and child activities, the parent and child activities being configured to use the added execution properties during execution, the added execution properties comprising corresponding identifiers that identify the added execution properties; an act of executing the parent activity including the one or more child activities within the modified execution context in the continuation based runtime, the modified execution context including the added execution properties that are available to the parent and any child activities during execution; and for the added one or more execution properties implementing an interface that (i) provides execution of a subtree of activities, and (ii) allows the parent activity to implement thread local storage setup and cleanup during execution of the subtree of activities.