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==================================
Extending Grammars and Inheritance
==================================
:warning: Unfinished
Another feature taken from OMeta is *grammar inheritance*. We can
write a grammar with rules that override ones in a parent. If we load
the grammar from our calculator tutorial as ``Calc``, we can extend it
with some constants::
from parsley import makeGrammar
import math
import calc
calcGrammarEx = """
value = super | constant
constant = 'pi' -> math.pi
| 'e' -> math.e
"""
CalcEx = makeGrammar(calcGrammar, {"math": math}, extends=calc.Calc)
Invoking the rule ``super`` calls the rule ``value`` in Calc. If it
fails to match, our new ``value`` rule attempts to match a constant
name.
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