The children of operations are called operands

BinaryOperation.leftChild → leftOperand
               .rightChild → rightOperand
UnaryIshOperation.child → operand

Done mostly with the following commands:

    find -name '*.ceylon' -exec grep -q leftChild {} \; \
        -and -exec sed -i 's leftChild leftOperand g' {} + \
	-and -exec sed -i 's rightChild rightOperand g' {} +

and

    find -name '*.ceylon' -exec grep -q child_ {} \; \
        -and -exec sed -i 's child operand g' {} +

and then lots of manual cleanup. I also found some leftover 'child's
with

    grep -r 'child[^r]' source
NarrowingTransformer: Remove source-gen help

No longer needed, as source-gen now inserts 'Precedence13Expression'
before 'Precedence1Expression'. (That’s still a problem, but not one
that can be fixed by this comment.)
Add NotOperation (RedHat’s NotOp)
Add tests for NotOperation backend conversion