code: @Override
  	public Constant getVal(String fldname) {
  		if (groupFlds.contains(fldname))
  			return groupVal.getVal(fldname);
  		if (aggFns != null)
  			for (AggregationFn fn : aggFns)
  				if (fn.fieldName().equals(fldname))
  					return fn.value();
  		throw new RuntimeException("field " + fldname + " not found.");
  	}
docs: Gets the Constant value of the specified field. If the field is a group
  field, then its value can be obtained from the saved group value.
  Otherwise, the value is obtained from the appropriate aggregation
  function.
  
  @see Scan#getVal(java.lang.String)