This function returns the weighted list of exponents of the leading
term of F, as determined by the first row of the weights matrix.
Thus, if all the weights are 1, this function returns the same thing
as Log(F).
Example
Use R ::= QQ[x,y];
F := x^2-y;
WLog(F);
[2, 0]
-------------------------------
Use R ::= QQ[x,y], Weights(2,3);
F := x^2-y;
WLog(F);
[4, 0]
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