Perform simple boolean (logical) computation.
Evaluate a plain boolean comparison storing the result into a variable:
eval <target-variable> <boolean-comparison>
Compute boolean and or or operation:
<boolean-instruction> <target-variable> <boolean-comparison>
Compute boolean not operation:
not <boolean-variable>
The <boolean-instruction> must be either and or or.
A <target-variable> is defined as the identifier of a <variable> whose
value will be set to 1 or 0 depending on the boolean result -
true or false, respectively - yielded by the boolean
comparison or operation.
A <boolean-variable> is defined as the identifier of a <variable>
whose current value is a string representing a number, the boolean value
of which will be considered false if the number equals 0, true
if the number equals 1, or invalid otherwise.
A <boolean-comparison> is defined as follows:
<comparison-variable><comparison-operator><comparison-value>
<comparison-variable>
A <comparison-variable> is the identifier of either a <variable>
or an <associative-variable>, possibly not set yet.
A <comparison-value> can be either a <variable>, or
a string of characters, possibly empty.
The <comparison-operator> must be one of the following characters:
With the exception of comparison operator ~, that always
implies a pattern-matching comparison between one string value and
a regular expression,
the type of comparison performed by Tefigel depends on the value of
<comparison-variable> and
<comparison-value>: if both values are numerical, then a numerical
comparison takes place, otherwise a lexicographical comparison
is performed.
A <boolean-comparison> with no <comparison-operator>
and no <comparison-value> is assumed to be as follows:
<comparison-variable>#0
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