public ProgramState executeOneStep(ProgramState program)
{
if (program == null)
{
throw (new InterpreterException("error: no program to execute"));
}
ExecutionStack <Statement> executionStack = program.getExecutionStack();
if (executionStack.isEmpty())
{
throw (new InterpreterException("\nerror: execution stack is empty"));
}
else
{
Statement statement = executionStack.pop();
return(statement.execute(program));
}
}