Removed the workaround for GCC bug that was fixed in 4.6.4.
Provided a specialization of leaf<> for callables. Thus, one can now assign things like lambdas to leaves.
Replaced busy_1_sec with a simpler lambda inside the main, near the point of use.
Added example for a tree with a leaf that is assigned a lambda.
Reverted all whitespace to tabs.
Sprinkled some override specifiers where it made sense.
Updated "Technical considerations" section of the documentation.