Get the lower triangular part of a matrix.

::

 tril(Sparsity a, bool includeDiagonal) -> Sparsity
 tril(DM a, bool includeDiagonal) -> DM
 tril(SX a, bool includeDiagonal) -> SX
 tril(MX a, bool includeDiagonal) -> MX

Extra doc: https://github.com/casadi/casadi/wiki/L_46

Doc source: https://github.com/casadi/casadi/blob/develop/casadi/core/sparsity_interface.hpp#L405

Implementation: https://github.com/casadi/casadi/blob/develop/casadi/core/sparsity_interface.hpp#L405-L407

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