Retrieves the 'start' variable of a task.
A cptask is represented by three cpvar:

• 'start' representing the start time of the task
• 'end' representing the completion time of the task
• 'duration' representing the duration of the task.

These three structural variables are linked with the following constraint: start + duration ≤ end.
The start time variable represents two specific parameters of the task: the Earliest Starting Time (EST represented by its lower bound) and its Latest Starting Time (LST represented by its upper bound). The graphic below illustrates these properties:

The cpvar representing the start time of t

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