title: Check if a record exists in a Julia array
body: <p>Say we have a matrix:</p>
  <pre><code>A = [1.0 2.0 3.0; 4.0 5.0 6.0] #2×3 Matrix{Float64}
  </code></pre>
  <p>and a record:</p>
  <pre><code>b = [1.0 2.0 3.0] #1×3 Matrix{Float64}
  </code></pre>
  <p>what is the most efficient way to check if record b exists in matrix A in Julia?</p>
  <p>Doing <code>b in A</code> returns false.</p>
  <p>And writing a nested for-loop when we may have a large matrix (many dimensions and many rows) to check seems inefficient.</p>