private void fact() throws IOException {
if (sta.lastToken() == TOKEN_ID) {
builder.buildOperand(sta.getString());
sta.nextTokenType();
} else if (sta.lastToken() == TOKEN_NUM) {
builder.buildOperand(sta.getInt());
sta.nextTokenType();
} else if (sta.lastToken() == TOKEN_OSB) {
sta.nextTokenType();
expr();
if (sta.lastToken() != TOKEN_CSB)
throw new InvalidExpressionError("Expected ]. Got '" + sta.lastToken() + "' instead.");
sta.nextTokenType();
} else
throw new InvalidExpressionError(
"Expected [, a number or an identifier. Got '" + sta.lastToken() + "' instead.");
}
