presto-parser/src/main/java/io/prestosql/type/TypeCalculation.java/CalculateTypeVisitor/visitIdentifier
class CalculateTypeVisitor:        @Override
        public BigInteger visitIdentifier(IdentifierContext ctx)
        {
            String identifier = ctx.getText();
            Long value = inputs.get(identifier);
            checkState(value != null, "value for variable '%s' is not specified in the inputs", identifier);
            return BigInteger.valueOf(value);
        }