the retail tax base is not restricted to receipts from sales of tangible personal property at retail, but rather comprehends all receipts from the seller's business activities unless certain other conditions are met, see State Tax Commission v. Holmes & Narver, Inc and City of Phoenix v. Arizona Rent-A-Car Systems, Inc Arizona Revised Statutes Annotated section 42-5008(A) (Supp. 1999) levies "transaction privilege taxes" measured by the amount or volume of business transacted by persons on account of their business activities, and in the amounts to be determined by the application of rates against values, gross proceeds of sales or gross income, as the case may be, as prescribed by this article and article 2 of this chapter. The "retail classification" of the transaction privilege tax "is comprised of the business of selling tangible personal property at retail." A.R.S. 42-5061(A). the rate for the retail classification is five percent of the "tax base." See A.R.S. 42- 5010(A)(1)(m) (Supp. 1999). the tax base equals "the gross proceeds of sales or gross income derived from the business." A.R.S. 42-5061(A). Section 42-5001(5) (Supp. 1999) defines "gross proceeds of sales" as "the value proceeding or accruing from the sale of tangible personal property without any deduction on account of the cost of property sold, expense of any kind or losses, but cash discounts allowed and taken on sales are not included as gross income." Because the term "gross income" is potentially more encompassing than "gross proceeds of sales" as pertains to services, we treat "gross income" as definitive of the retail classification's tax base. Arizona Revised Statutes Annotated section 42-5001(4) defines "gross income" as "the gross receipts of a taxpayer derived from trade, business, commerce or sales and the value proceeding or accruing from the sale of tangible personal property or service, or both, and without any deduction on account of losses." (Emphases added.) "Gross receipts" are defined as the total amount of the sale, lease or rental price, as the case may be, of the retail sales of retailers, including any services that are a part of the sales, valued in money, whether received in money or otherwise, including all receipts, cash, credits and property of every kind or nature, and any amount for which credit is allowed by the seller to the purchaser without any deduction from the amount on account of the cost of the property sold, materials used, labor or service performed, interest paid, losses or any other expense. Gross receipts do not include cash discounts allowed and taken nor the sale price of property returned by customers if the full sale price is refunded either in cash or by credit. A.R.S. 42-5001(7) (emphasis added). This definition of "gross receipts" applies to the retail classification only (with arguable exceptions not relevant here). See Ebasco Servs., Inc. v. Arizona State Tax Comm'n, 105 Ariz. 94, 97, 459 P.2d 719, 722 (1969) (interpreting former A.R.S. 42-1301, as amended by 1960 Ariz. Sess. Laws, ch. 21, 1, and 1968 Ariz. Sess. Laws, ch. 89, 84). Section 42-5061, which contains more than eighty exclusions or deductions from the retail tax base, provides as follows in subsection (A)(2): "The tax imposed on the retail classification does not apply to the gross proceeds of sales or gross income from . . . services rendered in addition to selling tangible personal property at retail." The tax is on the contracting business, not merely on the form of a series of contracts performed in the pursuance of that business. Here that business is two-fold: design and engineering, and construction. Where (1) it can be readily ascertained without substantial difficulty which portion of the business is for non-taxable professional services (design and engineering) (2) the amounts in relation to the company's total taxable Arizona business are not inconsequential, (3) those services cannot be said to be incidental 4 to the contracting business, the professional services are not merged for tax purposes into the taxable contracting business and are not subject to taxation.Arizona Rent-A-Car Systems, the only case to apply the Holmes & Narver test outside the prime contracting classification, did so in deciding whether income from concededly non-taxable services (sale of gasoline) was part of the personal property rental tax base in the Phoenix City Code. 182 Ariz. at 78-80, 893 P.2d at 78-80.