<#61702#>Figure: <#7395#>Beginning Student Scheme<#7395#>: The vocabulary (core)<#61702#>
The first category is that of variables, which are the names of functions
and values. The second introduces constants: boolean, symbolic, and numeric
constants. As indicated before, Scheme has a powerful number system, which
is best introduced gradually by examples. The final category is that of
primitive operations, which are those basic functions that Scheme provides
from the very beginning. While it is possible to specify this collection in
its entirety, it is best introduced in pieces, as the need arises.
For the classification of Scheme sentences, we also need three
<#61703#><#7397#>keywords<#7397#><#61703#>: <#61704#><#7398#>define<#7398#><#61704#>, <#61705#><#7399#>cond<#7399#><#61705#>, and <#61706#><#7400#>else<#7400#><#61706#>.
These keywords have no meaning. Their role resembles that of punctuation
marks, especially that of commas and semicolons, in English writing; they
help programmers distinguish one sentence from another. No keyword may be
used as a variable.