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       The VaR risk measure specifies a probability, Rho, e.g., in the interval [ 0.01, 0.1].
The VaR of a portfolio is the ending wealth level such that at most a fraction Rho of the outcomes
that might actually occur, have an ending wealth < VaR.
VaR is popular because it is relativiely easy to understand.
It is criticised for several reasons:
    a) It is expensive to compute optimal VaR portfolios when there are lots of scenarios.
    b) VaR is not a "coherent risk measure."  It may suggest that holding a portfolio of just one
bond with a 0.04 chance of default is better than holding a 50/50 mix of two independent
but equivalent bonds, each with a 0.04 chance of default (and say Rho = 0.05).
    c) it does not take into account long tails, 
        e.g., a low probability (<< Rho) of losing a huge amount of wealth.