- _x_ is an identifier that always denotes the mathematical free variable. It cannot be assigned.
- Sollya manipulates mathematical functions of a single variable. The first time that a variable name is used without having been assigned before, this variable name is automatically considered by Sollya as the name of the free variable. Subsequently, any other unassigned variable name will be considered as the free variable with a warning making this conversion explicit. This is convenient for an every-day use of the interactive tool, but it has the drawback that the free variable name can change from a session to another. There are contexts (e.g., within a procedure, or for doing pattern matching) when one might want to refer to the free variable regardless of its name in the current session. For this purpose _x_ is a universal identifier, always available and always denoting the free variable, whatever its name is in the current context.
- _x_ can be used to retrieve the name of the current free variable as a string by simply concatenating it to "".

> sin(a);

sin(a)

> b;

Warning: the identifier "b" is neither assigned to, nor bound to a library function nor external procedure, nor equal to the current free variable.

Will interpret "b" as "a".

a

> _x_;

a

> sin(y);

sin(y)

> f = proc(a) {

return sin(a + _x_);

};

> rename(y,z);

Information: the free variable has been renamed from "y" to "z".

> f(1);

sin(1 + z)

> match f with

sin(a) : { print("sin of a with a =", a);

match a with

_x_ : { print("a turns out to be the free variable"); }

default : { print("a is some expression"); };

}

_x_ : { print("Free variable") ; }

default: { print("Something else"); };

sin of a with a = y

a turns out to be the free variable

sin(wurst)

> varname = _x_ @ "";

> print("The current name of the free variable is " @ varname);

The current name of the free variable is wurst