title: What are the constraints for tensorflow scope names?
body: <p>I'm running a tensorflow model and getting the following error:</p>
  
  <p>ValueError: 'Cement (component 1)(kg in a m^3 mixture)' is not a valid scope name.</p>
  
  <p>I get that tf probably doesn't like special chars and spaces in its scope names, but I'm trying to find an actual doc on what chars are allowed. Does anyone know where I could find this?</p>
  
  <p>Thanks.</p>