public override string ToString()
{
if (str == null)
{
StringBuilder s = new StringBuilder();
append(s, "kg", kg); append(s, "m", m);
append(s, "sec", sec); append(s, "K", K);
append(s, "A", A); append(s, "mol", mol);
append(s, "cd", cd);
str = s.ToString();
}
return str;
}