## An overview of the mathematics of GRACE

Rigorous proofs for ruler and compass construction can be very subtle. In this section, we give a quick overview of the mathematical framework for the proof system used in GRACE.

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### Constructions

The fundamental building block of GRACE is the construction. A construction takes some points as inputs, and produces some outputs. There may be restrictions on the input shapes, called input constraints, that must be met before the construction can be applied. The construction may produce shapes as output, constraints as output, or both. For instance, the midpoint construction produces the midpoint as output, plus a constraint that asserts that the point is halfway between the inputs. The term "theorem" refers to a construction that produces no output shapes, and "axiom" refers to a construction for which no proof is provided. "Primitive" refers to an axiom that is built-in to the system. (such as "Line" and "Intersect".) The "Intersect" primitive is unique in that it is the only construction that may return a variable number of shapes, and it is also the only construction that accepts any shape other than points as input.

### Constraints

Proofs in GRACE involve constraints, which are symbolic mathematical statements regarding the relationship of points. There are two kinds of measurements - the distance between two points, and the angle between three points. Constraints are expressed as a linear combination of measurements with positive integer coefficients. Constraints do not include constant terms (except for PI in angle constraints).

Constraints are created as assumptions, as outputs from constructions, and automatically during intersection. (Constructions that use intersect may also create automatic constraints.) Any constraint that can be expressed as a linear combination of other constraints will also be recognized by GRACE as true.

In the first version of GRACE, measurements may only be in terms of points. You cannot, for instance, use the length of a line or the distance between two circles. Additionally, construction inputs may only be points, and intersections may only produce points.

### Constraints from primitive operations

Constructions in GRACE are created from a sequence of primitive constructions. These primitive constraints are of two types. The "Line", "Circle, "Line segment", "Ray" and "Complementary ray" constructions take a pair of points and create a curve. Points on these curves can be characterized by simple distance constraints. For example, a circle with center p and containing a point q is the set of all points r such that dist(p,q)=dist(p,r). The line segment with endpoints p and q is the set of all points r such that dist(p,r)+dist(r,q)=dist(p,q). GRACE associates such a constraint with each line or circle created.

### Handling sided-ness using rays and line segments

Traditional ruler and compass constructions (e.g Euclid) rely on a static diagram that encodes how the curves in the diagram intersect. The diagram also encodes subtle notions of sided-ness that are never explicitly stated. GRACE explicitly captures sided-ness using the linear primitives, "Ray", "Complementary ray" and "Line segment". For example, "Ray(p,q)" consists of those points r that are on the same sided of p as q. Given another point x, GRACE automatically treats the expressions angle(x,p,q) and angle(x,p,r) as being identical. This type of simplification is usually done without comment in traditional proofs. If it is noted, the reader is referred to the diagram.

### Failure of a construction

During the definition of a construction, the user may perform various intersection operations. The operations may have zero, one, or two points as their result depending on the curve primitives and geometric location of their input points. As the user drags the input points, the number of intersection points may change. Once an intersection is specified and its intersection points computed numerically, GRACE treats a change in the number of output points as a failure of the construction.

For example, suppose a construction uses the intersection of two circles. If these circles fail to intersect, then the construction makes little sense and should be viewed as having failed. One way of viewing this policy is that the user specifies the input configuration consistent with the intent of the construction. Radical changes in the location of the input points may result in a totally different construction.

Given a set of input and output constraints associated with a construction, a proof in GRACE asserts that if the input constraints are satisfied AND the construction succeeds (i.e. does not fail), then the output points are guaranteed to satisfy the output constraints. Note that proofs in GRACE make no guarantees about whether a particular construction succeeds. Proving that a construction always succeeds is often beyond the scope of GRACE.