Constraining mixed content models

[Gregory Murphy <Gregory.Murphy@eng.sun.com>]

Is there a schema language for XML that allows one to specify that an
element may have mixed content, but must not be empty or contain only
space characters? I would like to define a document type such that 

  <label>xxx</label>
  <label>x<y>x</y>x</label>
  <label><y/></label>

are valid, but

  <label></label>
  <label>   </label>

are not. 

// Gregory Murphy.  Isopaleocopria.