title: curve in R:'expr' must be a function, or a call or an expression containing 'x'
body: <p>I receive error drawing curve:</p>
  
  <pre><code>rho&lt;-function(t,k) ifelse(abs(t)&lt;=k,t^2,(2*k*abs(t))-k^2)
  plot(farm~land,data=farmland)
  curve(rho(k=19000),xlim=c(-10,10),col="blue", add="TRUE")
  
  Error in curve(rho(k = 19000), xlim = c(-10, 10), col = "blue", add = "TRUE") : 
    'expr' must be a function, or a call or an expression containing 'x'
  </code></pre>
  
  <p>How should I fix it?</p>