State Invariant
A StateInvariant 
is a condition, or constraint applied to a lifeline. The condition must be fulfilled for the lifeline to exist.
To define a StateInvariant:
1.Click the State invariant icon, then click a lifeline, or an object activation to insert it.
2.Enter the condition/constraint you want to apply, e.g. accountAmount > 0, and press Enter to confirm.
