Quantity Function

Function	Description
`quantity (truth-value-expression) = (quantity)`	Returns a quantity value for a truth-value-expression. Some examples are: `quantity (true) =1` `quantity (false) = -1.0` `quantity (fuzzy truth value) = a real number between -1.0 and +1.0 exclusive`
`quantity (text-expression) = (quantity)`	Extracts any numeric characters found at the beginning of text-expression and returns them as quantity values. Leading spaces are ignored; numbers may begin with a plus sign, a minus sign, or may be in exponential format (contain an e preceding an exponent of ten, by which to multiply the preceding mantissa). If no numeric characters are found at the beginning of text-expression, G2 signals an error. This function does not evaluate an arithmetic expression that may be embedded in the text value (the string enclosed in quotation marks). Some examples are: `quantity ("123yes no") = 123` `quantity ("-12.3abc") = -12.3` `quantity ("exp(2)") = error` `quantity ("abc 123") = error`