a missing fractionExpr should not always automatically equal 1
mixed expression bcs are defined with up to three expressions:
- valueExpr
- gradientExpr
- fractionExpr
Missing values:
- missing valueExpr treated as zero
- missing gradientExpr treated as zero
- missing fractionExpr treated as one
Treating a missing fractionExpr like "1" makes sense for handling value expressions (when gradientExpr is missing), but does not really make sense when valueExpr itself is missing. For example,
{
gradientExpr " .... ";
}The user would rightly expect that the gradient expression should be used, not ignored.