When the **–reactive** option is set for **tlcore**, then the application becomes reactive, and the set of equations is allowed to vary over time, and the expressions being evaluated can refer to the **time** dimension. The idea is that at each instant,

- The header may be added to.
- The set of equations may be modified.
- The set of expressions to be evaluated may be modified.
- Then the expressions for that instant are evaluated.

When **tlcore** is reading from the standard input, then the input will be of the form

*header*

%%

*equations*

%%

*expressions*

$$

*header*

%%

*equations*

%%

*expressions*

$$

...

Each instant corresponds to the lines appearing between successive occurrences of **$$**.

The output is of the same form as the input, with occurrences of **$$** to separate the different instants.

When the input is given in the form of option **—input=***infile*, then the entire input, for all instants, is read from *infile*. When the input is given in the form of option **–inputiter=***inprefix*, then the input for instant *n* comes from file *inprefix*_{n}. In other words, the input for each instant is placed in a different file.

Similarly for **–output** and **–outputiter**.

If the **–uuid** option is set, then the universal identifiers for the equations are presented to the programmer, who can then manipulate them explicitly, with lines like the following two:

delete *eqnuuid*_{1};;

replace *eqnuuid*_{2} *equation*;;

For the first line, if **#time**, the time that this line is added to the equations, has value *t*, then from *t* on, *eqnuuid*_{1} is no longer usable. For the second line, the equation corresponding to *eqnuuid*_{2} is replaced, from *t* on, with *equation*. If the value of **#time** is changed to *t’*, which happens to be less than *t*, then all of the equations available at time *t’* continue to be accessible.

Changes to the set of equations are done synchronously. All additions, deletions and replacements for a given instant are done as a single transaction, and must be consistent.

An empty instant stops the **tlcore** program.

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