Restrict one class can learn skill from only one other class

[chuong2a]

L2J Revision 5685:
L2JDP Revision 9261:

How to restrict one class can learn skill from only one other class?
When enable AltGameSkillLearn in file Character.properties, so a char can learn all skills from all class. But this will get boring.
# When enabled, the following will be true:
# All classes can learn all skills.
# Skills of another class costs x2 SP to learn.
# Skills of another race costs x2 SP to learn.
# Skills of fighters/mages costs x3 SP to learn.
AltGameSkillLearn = True