Subtract two FExprs, which corresponds to python operation
x - y.
y are multi-column expressions, then they must have the
same number of columns, and the
- operator will be applied to each
corresponding pair of columns. If either
y are single-column
while the other is multi-column, then the single-column expression
will be repeated to the same number of columns as its opponent.
The subtraction operation can only be applied to numeric columns. The
resulting column will have stype equal to the larger of the stypes of its
arguments, but at least
The arguments must be either
FExprs, or expressions that can be
An expression that evaluates
x - y.