code: @Override
  	public boolean hasField(String fldname) {
  		if (groupFlds.contains(fldname))
  			return true;
  		if (aggFns != null)
  			for (AggregationFn fn : aggFns)
  				if (fn.fieldName().equals(fldname))
  					return true;
  		return false;
  	}
docs: Returns true if the specified field is either a grouping field or created
  by an aggregation function.
  
  @see Scan#hasField(java.lang.String)