title: Sympy custom function
body: <p>I noticed, that there is no functions like sech(x) and csch(x).</p>
  
  <p>Is there any way to quickly define them as 1/cosh(x) and 1/sinh(x) respectively?</p>
  
  <p>Also, how can I make sympy to treat arccos as acos?</p>
  
  <p>I'm using parser, so 'cos(pi/2)' is correctly parsed. But I want 'arccos(pi/2)' to be parsed in a similar manner.</p>