   <span class="sd">&#39;&#39;&#39;approximately solves min_x |x|_0 s.t. Ax=b using Orthogonal Matching Pursuit</span>
<span class="sd">        Args:</span>
<span class="sd">            A: design matrix of size (d, n)</span>
<span class="sd">            b: measurement vector of length d</span>
<span class="sd">            tol: solver tolerance</span>
<span class="sd">            nnz = maximum number of nonzero coefficients (if None set to n)</span>
<span class="sd">            positive: only allow positive 