[Pyparsing] fourFn.py explanation and slight modification

[Glenn P. <gle...@gm...>]

Hi I am new to pyparsing and I am having trouble understanding the grammer
of the example fourFn.py
<http://pyparsing.wikispaces.com/file/view/fourFn.py/30154950/fourFn.py>

I am trying to add comparison operators and logical operators to this
example. The comparisons work ok but I not sure how to add the logical
operators AND OR NOT

So far I have this

        plus  =  Literal( "+" )
        minus =  Literal( "-" )
        mult  =  Literal( "*" )
        div   =  Literal( "/" )
        lte   =  Literal( ">=" )
        lt    =  Literal( "<" )
        gt    =  Literal( ">" )
        anding = CaselessLiteral( "and" )
        lpar  =  Literal( "(" ).suppress()
        rpar  =  Literal( ")" ).suppress()
        addop  = plus | minus | lte | lt | gt
        multop = mult | div
        expop = Literal( "^" )
        pi    = CaselessLiteral( "PI" )
        sensor_values = CaselessLiteral( "sensor_values" )
        expr = Forward()
        atom = ((Optional(oneOf("- +")) +

 (sensor_values|pi|e|fnumber|ident+lpar+expr+rpar).setParseAction(self.pushFirst))
                | Optional(oneOf("- +")) + Group(lpar+expr+rpar)
                ).setParseAction(self.pushUMinus)
        # by defining exponentiation as "atom [ ^ factor ]..." instead of
        # "atom [ ^ atom ]...", we get right-to-left exponents, instead of
left-to-right
        # that is, 2^3^2 = 2^(3^2), not (2^3)^2.
        factor = Forward()
        factor <<= atom + ZeroOrMore( ( expop + factor ).setParseAction(
self.pushFirst ) )
        term = factor + ZeroOrMore( ( multop + factor ).setParseAction(
self.pushFirst ) )
        expr <<= term + ZeroOrMore( ( addop + term ).setParseAction(
self.pushFirst ) )


Could someone perhaps explain the three lines below and how I would add my
 CaselessLiteral( "and" ) boolean
so I could do something like
nsp.eval('10 >= 5 AND 10 < 15')

factor <<= atom + ZeroOrMore( ( expop + factor ).setParseAction(
self.pushFirst ) )
term = factor + ZeroOrMore( ( multop + factor ).setParseAction(
self.pushFirst ) )
expr <<= term + ZeroOrMore( ( addop + term ).setParseAction( self.pushFirst
) )


PS what is the <<= operator ? I can't find the docs for it

Thanks for any help.