+/** Substitute symbols in expression and return the result as a new expression.
+ * There are two valid types of replacement arguments: 1) a relational like
+ * symbol==ex and 2) a list of relationals lst(symbol1==ex1,symbol2==ex2,...),
+ * which is converted to subs(lst(symbol1,symbol2,...),lst(ex1,ex2,...)).
+ * In addition, an object of class idx can be used instead of a symbol. */