Ratio expressions have the form 'x : y' (the ':' operator has the
highest precedence for a binary operator, and is left-associative).
Given a reference value 'a', the expression evaluates to
'a * (x / y)'.

Ratio expressions can be controlled by the caller by:

  - Enabling or disabling them:  They're meant to be used when the
    eevl servers two paired entries, and can be disabled otherwise.

  - Setting the reference value:  That's normally the value of the
    "other" entry of the pair--the one not currently being
    evaluated.

  - Inverting the ratios:  Normally, one entry refers to the
    antecedent term of the ratio, and the other entry refers to the
    consequent term of the ratio.  When evaluating the latter one,
    the ratio should be inverted.