nd X3 are hydrogen atoms, and one of X1, X2 and X3 is a group represented by the formula: RCO-, wherein R is defined above; ##STR9## wherein X4, X5 and X6 each represents a hydrogen atom or a group represented by the formula: RCO-, wherein R is as defined above; one of X4, X5 and X6 is a hydrogen atom, and two of X4, X5 and X6 are each a group represented by the formula: RCO-, wherein R is as defi