Enables the definition of arbitrary distance measures using a Java snippet. A distance measure is defined on two equal structured rows. You can refer to a cell value by enclosing the column name prefixed by either `row1` or `row2` for example `$row1.c1$` .

**Examples:**

Compute the Euclidean distance of the columns *c1* and *c2* :

`sqrt(sqr($row1.c1$-$row2.c1$)+sqr($row1.c2$-$row2.c2$))`

Compute the arithmetic mean of the Levenshtein distance of the columns *c1* and *c2* :

`mean(levenshtein($row1.c0$,$row2.c0$),levenshtein($row1.c1$, $row2.c1$))`

Note that strings which are part of the expression and are not from the input data (or the result of another wrapped function call) need to be enclosed in double quotes ('"'). Additionally, if the string contains a quote character, it must be escaped using a backslash character ('\"'). Finally, other special characters such as single quotes and backslashes need to be escaped using a backslash. For instance, a single backslash in a string is written as two consecutive backslash characters; the first one acts as the escape character for the second.