title: Error in terms.formula(formula) : '.' in formula and no 'data' argument
body: <p>I'm tring to use neuralnet for prediction.</p>
  
  <p>Create some X:</p>
  
  <pre><code>x &lt;- cbind(seq(1, 50, 1), seq(51, 100, 1))
  </code></pre>
  
  <p>Create Y:</p>
  
  <pre><code>y &lt;- x[,1]*x[,2]
  </code></pre>
  
  <p>Give them a names</p>
  
  <pre><code>colnames(x) &lt;- c('x1', 'x2')
  names(y) &lt;- 'y'
  </code></pre>
  
  <p>Make data.frame:</p>
  
  <pre><code>dt &lt;- data.frame(x, y)
  </code></pre>
  
  <p>And now, I got error</p>
  
  <pre><code>model &lt;- neuralnet(y~., dt, hidden=10, threshold=0.01)
  </code></pre>
  
  <blockquote>
    <p>error in terms.formula(formula) : '.' in formula and no 'data'
    argument</p>
  </blockquote>
  
  <p>For example, in lm(linear model) this is worked.</p>