def sdfSetNoChangeProp(mol, sdfprop, sdfvalue):
	"""
	sdfSetNoChangeProp() set property sdfprop to value sdfvalue for molecule mol.
	If the sdfprop is already defined in mol, the value is not changed.
	returns a pair made of Boolean and of the new molecule.
	The Boolean is False only if the property sdfprop was already defined in mol 
	"""
	sdfkeyvals = mol["keyvals"]
	sdfkeys = [pair[0] for pair in sdfkeyvals] if sdfkeyvals else []
	if sdfprop not in sdfkeys:
		newsecond = ">  <"+sdfprop+'>'
		if sdfvalue:
			newsecond += '\n'+sdfvalue
		newpair = (sdfprop, newsecond)
		sdfkeyvals.append(newpair)
		mol["keyvals"] = sdfkeyvals
		return (True, mol)
	else:
		return (False, mol)