This is t1-template.txt

With $(a) & $(b):
This is t1-include.txt 
  a = default value used when a is undefined
  b = default value used when b is undefined
End of t1-include.txt

On defining a=aaa & b=bbb:
This is t1-include.txt again
  a = aaa
  b = bbb
End of t1-include.txt

On setting a="aa":
This is t1-include.txt again
  a = "aa"
  b = bbb
End of t1-include.txt

End of t1-template.txt