ject(A, y, subset_number)</span><span>.</span></div><div  class = 'S1'><span style=' font-weight: bold;'>Extracting the system matrix</span></div><div  class = 'S1'><span>The class object can also be used to form the system matrix itself, either the entire system matrix or a subset. However, to form the system matrix, the class object has to be created with implementation 1 (e.g. </span><span style=' font-family: monospace;'>options.implementation = 1</span><span>). The matrix itself is obtained with </span