A left-to-right associative paste operator.
This macro avoids the ambiguity of the
P99_PASTE(0.1E, -, 1)
is guaranteed to produce the token
ETSAP(0.1E, -, 1)
is not valid: the intermediate operation to paste tokens `-' and `1' would result in an invalid token and is thus rejected.
This macro does the evaluation of the arguments first and then proceeds at the concatenation of the results.