  PENLAB is an open source software package for nonlinear optimization, linear
and nonlinear semidefinite optimization and any combination of these. It is
written entirely in MATLAB. PENLAB is a young brother of our code PENNON
\cite{pennon} and of a new implementation from NAG \cite{naglib}: it can solve
the same classes of problems and uses the same algorithm. Unlike PENNON, PENLAB
is open source and allows the user not only to solve problems but to modify
various parts of the algorithm. As such, PENLAB is particularly suitable for
teaching and research purposes and for testing new algorithmic ideas.
  In this article, after a brief presentation of the underlying algorithm, we
focus on practical use of the solver, both for general problem classes and for
specific practical problems.
