code: public static Component of(final String pdbccId,
  			final boolean isNTerminal, final boolean isCTerminal) {
  		return of (Collections.singleton(pdbccId), isNTerminal, isCTerminal);
  	}
docs: Get or create a Component.
  @param pdbccId Protein Data Bank ID.
  @param isNTerminal true if occurring at N-terminal. false, otherwise.
  @param isCTerminal true if occurring at C-terminal. false, otherwise.
  @return a component.
  @throws IllegalArgumentException if pdbccId or type is null,
  or the pdbccId has been registered as a different type,
  or terminal condition is indicated for non-amino-acid component,
  or both N-terminal and C-terminal are true.