code: @Override
  	public String onVariable(String variable) {
  		String value = null;
  		if(variables != null && variables.containsKey(variable)) {
  			Object val = variables.get(variable);
  			value = (val != null) ? val.toString() : null;
  		}
  		return value;
  	}
docs: Returns the String representation of the value corresponding to the given
  key in the map (if available), null otherwise.
  
  @see org.dihedron.core.variables.ValueProvider#onVariable(java.lang.String)