Grammar for a system of ODEs. I cheat a bit with formula by adding the unecessary <formula> <operator> <variable> and <variable> <operator> <formula>, it helps grammatical evolution algorithms find simpler formulas.

gistfile1.txt

<expr>      ::= dS/dt = <formula>\ndI/dt = <formula>\ndR/dt = <formula>
<formula>   ::= <unary><formula> | <formula> <operator> <formula> | <formula> <operator> <variable> | <variable> <operator> <formula> | <variable>
<unary>     ::= -
<operator>  ::= - | *
<variable>  ::= a | b | I | S